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Which folders are useless and which if them are important in ReactJS?

I ran this command to create a ReactJS app
npx create-react-app learningapp
This created several folders now, I dont know which of them are important. I mean I dont know their purpose.
Can anyone explain their purpose in short?

node_modules -- very important, as this will contain all the npm packages and their entire list of dependencies installed.
public -- very important, contains the static files served by your web server.
Index.html -- the index.html file where your react app will inject elements into. I believe this is the only "essential" file.
The other files in this folder will contain logos and manifests if you'd like your webpage to be able to be installed as a mobile app seamlessly. The manifest.json file holds the information about what the app icon and such will look like.
Favicon is the tiny logo you see in your tab title
robots.txt will have the instructions for bots visiting your website. Read about it here if you'd like (https://www.cloudflare.com/learning/bots/what-is-robots.txt/)
src -- very important, will contain your source code. If you want your app to do anything at all, it wouldn't be very wise to delete this folder. If you want to rename this to something else, you can, but you'd have to mess with the webpack configurations. So, not worth the little extra effort. However, you may alter the folder contents.
Unless you want performance monitoring and are writing tests for your app, you can safely delete the test file and the report webvitals stuff. You can make your test files somewhere else too, it doesn't matter if it's here. Just make sure you configure your testing library so that it looks for the correct files.
The rest of the files in this folder can be modified all you like, but try not to touch index.js unless you want to go mess with the webpack configs to change the entry point. Webpack looks for index.js as an entry point to build its dependency tree during compilation.
.gitignore -- this is the files/folders you can tell git to ignore when tracking your folder. A usual candidate for this file is the node_modules folder.
package.json/package.json -- very important, don't directly mess with these unless you know what you're doing. This contains the info about npm packages which your require to run your project properly. A situation where you will need to mess with package.json is when you want to add some custom npm scripts, which is often quite useful.
README.md -- just your readme file which is used to display info about the project on your github repo for example. You can delete it, but just put something on there containing basic info about the repo/ what it does.

Import JS file inside node_modules in Angular

I have an angular app, to this I am trying to add an npm package called svgedit. It gets installed. But when I try to reference a java-script file it says
net::ERR_ABORTED 404 (Not Found)
This is how I am trying to reference it in my index.html in the head section,
<script type="module" src="node_modules/svgedit/svgedit-config-es.js"></script>
I can access this file from within vs code by clicking "ctrl + left mouse". It opens the file properly meaning the reference is proper. But when it runs it gives the above mentioned error in the console. I have also tried switching the type attribute to "text/javascript".
Basically I am trying to integrate this package in my angular app. What is the correct method to go about it.

ng serve will only serve compiled files from your src directory and what''s in the assets folder. It will not serve node_module files directly.
The usual way to add scripts it to add it to the scripts section in angular.json
scripts:
[ //other scripts
"node_modules/svgedit/svgedit-config-es.js"
]

Is your html page in the root directory of your project? if not make sure to navigate to the file correctly. if the html page in the root directory of your project then there must be a typo in the src

Basically I am trying to integrate this package in my angular app. What is the correct method to go about it.
The right way to go about it is to use a bundler like webpack, rollup etc...

loading html from scriptJS

I am using VueJS, after compiling code in the production mode. I have 1 index.html and some assets. The assets I upload all to the cloud.
The problem I'm facing is the client doesn't accept the HTML file. They require only 1 line to load all the things
Is there any package npm support for that?

Maybe you are looking for vue-custom-element package. This will give you an ability to define custom element which you can use as AdTag after build.
Check this out: Vue Custom Element

How to enable hot reloading for the build folder in create-react-app?

I have a React application created using create-react-app. I also have an external application that is using this application by including the bundled JS and CSS files (the ones create using the build script).
Right now, I'm using the React template created by the dotnet new command, and I have configured the server to serve static files located in the build folder.
For this reason, I have created a couple of additional NPM scripts that rename the bundle files (remove the hash from the name), so that I don't need to update my external application's links with every build.
Right now, whenever I change something in the code, the whole build process has to be processed to create the two files.
I have created a "watch" task to run my build scripts whenever I have a change, but I was wondering if there is a way to speed up the process and somehow configure the React application to be served from memory or something just like when we "normally" run the application.
So, maybe a couple of questions:
How to achieve this in a "normal" React app created by create-react-app?
how to achieve this in the context of the dotnet template?

you can use nodemon in the build folder of the create react app.
as the documentation says
By default nodemon monitors the current working directory. If you want to take control of that option, use the --watch option to add specific paths:
so you can use something like this:
nodemon --watch app --watch libs app/server.js
for details try other approaches given here

This could be due to your filesystem, file extensions or the Create-React-App default webpack/project configuration. You don't necessarily have to change all of this because hot-reloading is supposed to work out of the box, and more so if the project has just started.
For example, I once had an issue with a Typescript installation(^17.0.1) where some files with extension .ts will not trigger hot reloading. I had to change to .tsx and add a React import. The same could happen with .js and .jsx files.
In case of problems with your filesystem (Unix, Mac) you can try the React config (FAST_REFRESH=false) here... or changing folder names, but I haven't bumped much into this.

laravel app.js very large file size

I have deployed a Laravel 5.3 application to Heroku. However, when loading /login, I noticed a very slow page load time. The problem seems to be a very large app.js file: /js/app.js. Here is a screenshot of the Network resource panel in DevTools: screenshot- Network panel. The 3rd resource from the top is the offending file.
I am not sure why this file has gotten so large. here is a link to the repository: https://github.com/AshMenhennett/Salon-Pricing.
I wasn't able to post anymore links, so do let me know if you would like direct links to specific files.
What should I be doing to mitigate this issue?

The most obvious thing you can do is to run npm run prod. This will compile the assets for production use. But in most cases, you must be looking at other solutions beyond running npm run prod. If your production file is too large, you must check your dependencies. Remove unnecessary dependencies and ensure that you don't use a lot of external libraries. For example, if you are using bootstrap, you should rely on Bootstrap's alerts in order to show alerts rather than using a Vue package to show alerts. I admit that sometimes you will need to use an external library to make your website interactive but to achieve that, you will have to sacrifice the performance. So your best bet in order to reduce the app.js file is to use the minimal external dependencies in your package.json.
The second thing you can do is use minimum HTML in your components' templates. A lot of components with heavy HTML/CSS will contribute to a larger app.js file. This is yet another approach that will result in a smaller app.js file.
Lastly, you should consider using Vue's component slots to pass HTML contents to your components. This will leave the HTML in your static files and only javascript data (API calls, props, etc.) will be compiled in the app.js file. This is an effective approach to build a smaller app.js file.
Edit: You can remove JQuery and Bootstrap scripts from the bootstrap.js file and can include these dependencies separately. It is always a good idea to have a few more scripts rather than having a very large script. i.e. browsers do parallel downloading and thus using JQuery and Bootstrap dependencies separately is a good idea.

From the looks of your link you've not created a production version of your assets, and currently all the source maps are in your app.js file, which will be adding a lot of the file size, the css and js output are also not compress/minified either.
Assuming you're using laravel elixir, you just need to run gulp --production and this will remove the source maps, compress the js and css outputs, etc.

For people that are using Laravel Mix you just need to run npm run prod to compress and remove source maps from app.js itself.

You need to load the components asynchronously
Webpack has an awesome feature to create chunks of code. The key to this is to use async components. These components get loaded completely asynchronously whenever the component is present on the page you just loaded.
Let's do it.
In resources/js/app.js
I changed
Vue.component('jobs', require('./pages/employer/jobs/Index.vue').default);
To
Vue.component('jobs', () => import('./pages/employer/jobs/Index.vue'));
and in webpack.mix.js
mix.webpackConfig({
output:{
chunkFilename:'js/vuejs_code_split/[name].js',
}
});
Now by running npm run watch or prod each component file is saved public/js/vuejs_code_split/[name].js
And the main app.js is automatically calling those components when required.