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What is SSG (Nuxt.js, Gatsby) doing?

I have heard SSG generates static sites.
Then I thought SSG generates pure HTML that didn't include React, but I think it may not be true now.
I think:
SSG generates a usual React App and rendered HTMLs for initialization.
As it is a usual React App, if I click a button and trigger a side effect, client-side rendering will be triggered and the page will be updated.
When routing using router is triggered, next page's js file and data obtained when build will be downloaded, and then client-side rendering will be triggered.
The next page's rendered HTML for initialization isn't used here.
Is it true?

SSG (Static Site Generators) like Gatsby and Next, what they do is to create an output HTML based on a React environment code. This doesn't mean that the site is "static" in terms of interaction. This means that the page you are requesting is already created so you are avoiding response and compilation time in the server.
Summarizing, given a "traditional"/"old-fashioned" PHP site. When you request the homepage, for example, your requests go to the server, the server transpiles the PHP into HTML (what the browser can parse and print) and then you get the page. That processing time is omitted in Gatsby/Next because the HTML is already created.
When you build your site in Gatsby/Next, the data is being retrieved from the sources (using GraphQL from markdowns, CMSs, APIs, JSONs, etc) and creates the output (that's why there's a /public folder generated). All your JavaScript and React is bundled into the output HTML so your website will be "dynamic" in terms of user interactivity, React is part of the ecosystem so your side-effects (triggered by useEffect hook for example) or your rehydration process (useState hook for example) will be part of your site.
Explained as:
When you navigate into another page, you are requesting a page that is already built and generated, that's why is so blazingly fast.

I've answered this question few weeks ago on the Nuxt discussions: https://github.com/nuxt/nuxt.js/discussions/9493#discussioncomment-948643
Let's say that SSG bring several things:
SEO
speed
ecology
[probably some other things]
There are several ways of doing SSG and all of them have their pro/cons and their use-cases. For the most part, and if you're using Nuxt.js, you will probably go the target: static, ssr: true route.
This will:
generate fully static pages during build time and you'll be able to host it on Netlify, Vercel or alike
hydrate the static content with some JS after you have fetched the static files
have the Vue behavior afterwards, as a classic SPA (hence managing the routing without further server calls)
This behavior is called Isomorphic or Universal, more info in the linked discussion.
Gatsby and Next.js do work in somewhat similar ways. There are some minor differences but the general is globally the same across those 3 AFAIK.
SvelteKit and Astro handle this a bit differently. May be interesting to give it a look!

How to use a NUXT component in another site

I have a working NUXT application with various pages and components in universal mode. Now I need to render one of the components on another static html site.
I can easily export a regular Vue application's component just adding a bundle script and div element to which the components renders.
But how can I do it with NUXT?

Nuxt is not really meant for a quick plug (with a script tag) but for an SSR usage (with some NodeJS build), so I highly doubt that you can make this. Or at least, I don't really see the point if you only use it as an SPA component.
If somebody knows a solution to make it work, I'm all yours on your opinion on this.

Creating a Javascript bundle with a few components for reuse

I have a backend rendered page (django in case it matters) which I want to soup up a little using some components from PrimeVue and a markdown editor packaged as a vue component. Right now, we have a few small animations using jquery for which we include jquery from a CDN directly into our pages. A few months ago, we needed to spice up a page using some more client side interactivity and we included vue.js via a CDN onto that page (dropping jquery) and then wrote some javascript in an index.js that we also loaded up from a CDN and got our work done. This is the current state of affairs. The page currently looks like this
<html>
....
<script src="https://cdn/vue.js"></script>
<script src="/static/index.js"></script>
The div #mainvue is where vue runs and does what it needs to.
This is where we are now.
Using plain vue is okay. Now, I'd like to throw in a few components from primevue as well as a 3rd party markdown editor that's wrapped as vue component. I want to bundle all of these as wel as plain vue itself into a single javascript bundle that I can throw onto a CDN and include into all my pages. Then my devs can do their day to day work in the index.js.
Is this a reasonable approach and if so, how do I do it? I'm not familiar with the javascript ecosystem. If not, what's the right way to solve this problem. I don't want to go all the way SPA and REST API. I just want to use a few 3rd party components and vue on a simple otherwise backend rendered page.

Since you mention you don't want to "go all the way SPA," a reasonable hybrid to is to use Vue in MPA (multi-page app) mode. This will require using a vue-cli/webpack configuration to compile your Vue components into bundles, but once you have this build pipeline, these bundles can used in individual Django templates via django-webpack-loader. Information can be passed from Django via template variables directly as Vue component properties.
Re bundling, yes you can bundle all these resources into a single JS using this method, but it's nearly as easy (and far more performant) to create one or more common bundles that represent shared logic (third party libs, invidual components, even Vue itself) and then pick and choose from among these bundles as needed on individual Django templates.
The steps to implement are a bit too involved to post directly here, but I've written a series of articles Django + Vue -- Best of Both Frontends that explains. There's also a cookiecutter for boostrapping new projects using this method. I realize you already have a site, but you can perhaps adapt the implementation there.
Good hacking!

Gatsby's Client Side Routing - What Gatsby features do my client-rendered pages still benefit from?

I built a small site that uses gatsby for static content, but then for some content that needs to be rendered on the client-side, I'm using client-only routes in gatsby.
I am not sure I fully understand how this works though - Say I have a Header, Footer & a font that I am using in my static site. On my client-only routes, I am using the same Header, Footer & font. Will I benefit at all from having used these elements in my statically components previously? Is the font being loaded anew, for example?
Basically, I would like to know what Gatsby-features my client-site content is losing out on now, and what I should maybe attend to a bit more, since Gatsby won't be handling this for me anymore. Especially in terms of pagespeed.

Yes you should benefit from having used those components and fonts already.
The React components that are being re-used will already be in a JS bundle that you have shipped to the user and shouldn't need to be fetched again. Likewise with the font files - but these will be asset files - not in a JS bundle.
The best way to see what is being fetched will be to test it out in a browser.
Load a static page
Open the Network tab in dev tools
Navigate to a client-only page and check for network activity
While those assets shouldn't be fetched twice I can imagine some instances where an incorrect setup would fetch them twice - so best to just double check.

Will I benefit at all from having used these elements in my statically
components previously?
The answer is yes. Gatsby works with #reach/router under the hood so you will have all benefits of it no matter if you use client-only routes or not.
In other words, the trickiest part of using client-only routes is the internal routing, for your site, in that scenario, Gatsby will handle the routing internally since it extends from #reach/router, so the shared components (header, footer, etc) will only be rendered on-demand and will be shared across your site, no matter if it's a client-only route or a static page.
I would like to know what Gatsby-features my client-site content is
losing out on now, and what I should maybe attend to a bit more, since
Gatsby won't be handling this for me anymore. Especially in terms of
pagespeed.
Summarizing a lot, when a page loads, #reach/router looks at the path prop of each component nested under <Router />, and chooses one to render that best matches window.location. So you will only render the needed code on-demand.
In terms of page speed, your site won't be affected because the site keeps being "static" and pre-mounted once the build is done. The only "negative" part of using client-only routes (if you want to say that) is the SEO part since they won't be intercepted by Google, but, that's the reason why you are using client-only paths, in most of the cases you don't want to index those pages.

Bundling react components based on per-component permission control before they are transpiled and shipped to client

I am developing an involved web app with asp.net core.
I am developing React components, writing all of my components with ES and JSX syntax.
I run webpack to transpile all of my code (so now I have pre-transpiled files ready to be served)
When a request comes in, I just serve my pre-transpiled bundles.
I wanted to have a way of only bundling and sending user-specific components (based on a list of features they have access to) to the client.
The only way I could figure doing this is to do "on-the-fly permission-controlled component bundling combined with on-the-fly jsx compilation" to serve my components.
I gather that webpack shouldn't be used as an on-the-fly bundler like this, so that is out of the picture...
Partial scrappy solution I came up with:
Using no importing or export mechanism in my js, I use Razor to cycle through my feature list, adding the appropriate (mostly modular) components in what I call "Dependency First Order" to the page, and at the end of each components' code, I write
class ComponentA extends React.Component { //Component Code Here }
window.ComponentA = ComponentA;
So all components are global and can be rendered.
This way, I am able to select what Components get sent to the client with Razor.
NOW, remember when I said "mostly modular"? Well if I am rendering a component within another component that the user doesn't have access to, this partial solution would leave the render statement embedded in main component that is rendering the sub-component itself, without the component code it's supposed to render actually being there. This being a dirty partial solution, I would just suppress the error if the component was non-existent and move on.
Bottom line is I am having a real difficult time making my react components 100% modular and being able to control the granularity of my 'component dependencies' so that no code is on the client that a user shouldn't have access to.
Ridiculous solution someone offered me:
It is also certainly out of the question that I would generate a set of bundles for every user and whenever an admin changes what a user has access to, I would re-render that bundle with webpack. (especially since I am dealing with thousands of users here).
As I am writing all of this, the more and more I feel like I am just being a perfectionist and should just go with the above paragraph.
The solution I should probably go with:
There is the ideology out there to just send all of your js to the browser and then selectively render them based on the permission of the user. Any security loopholes here would just be handled by server-side access control to lock down endpoints if a specific user did try to forge requests to parts of the application they don't have access to (which would be implemented regardless).
I am under the gun here and feel like I am overthinking most of this. I would be greatly appreciative of any feedback. Thank you.

It is possible to ship permission based JS bundle to client. You can leverage webpack dynamic import logic to load only required features JS bundles.
You need to create directory structure based on features and load them based on user permissions. Basically what webpack does is, it creates separate bundle for each feature and load it via dynamic import when requested.
Solution here 👇
Note: You might not see lazy bundles in codesandbox.io network panel, but, you can download project and run server locally to see bundles being lazy loaded.