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Could HTML file be reused dynamically?

If a website needs to have a page for every item, is it better to manually create pages with the same HTML code but different titles/images/descriptions or create only one but add content through javascrpit depending on a page a user followed, like that?
linkBtn.addEventListener('click', function() {
contentEl.innerHTML = `<div>
<div>
<h3>${title}</h3>
</div>
...etc
</div>`
})
or is there less horrible solution?

It is entirely up to you. Compare the stackoverflow website with the gmail one. Stackoverflow reloads the whole website as you navigate between pages. This means that your browser is requesting a new resource and the stackoverflow servers are returning that resource, possibly creating it dynamically with new question etc., but then just sending you raw HTML.
On the other hand, gmail loads once, but then fetches each different page entirely through javascript. This could involve asking the gmail server for new messages, but could also be just reworking the HTML to show a settings page, for instance.
There are obviously advantages and disadvantages to both ways of doing things.
As a side note, in javascript it is not a great idea to assign to innerHTML as this requires that the browser does a lot of work to re-parse the new markdown. Instead you should fully use the DOM model with functions such as document.createElement and Element.appendChild.

You need not create different pages for each product. Rather use HTML as template.
You can use something like handlebars to make templates.

I believe you should create one UI and fill it with data from some REST API. Look into creating dynamic websites, it is basic stuff.

Registering script for event validation from within the JS file

Just a quick question here. I am trying to register a js file for script validation using
if (!Page.ClientScript.IsClientScriptBlockRegistered("strIncludeJSFile")) Page.ClientScript.RegisterClientScriptBlock(strIncludeJSFile.GetType(), "strIncludeJSFile", strIncludeJSFile);
code in C# and it works well for the js files. But, some js files are used in multiple pages, so I am unsure if the above code will be a good idea. As such, I want to do the same thing in the js file itself, instead of using the code behind. Is there any possibility to do that? Or is this thing specific to C#?

of course you can register a js file (or script block) from html by using a script tag. However, the main reason RegisterClientScriptBlock exists is because developers might need to generate (or modify) a script block dynamically from code behind or conditionally register a script file dynamically.
If you need any of the above...generate a script block dynamically, then you "might" be better off registering it from code behind, I mean, it depends on the whole solution itself, it's hard to recommend an approach without having some context. Either way, some options are:
register the script blocks from code behind if you need to generate based on some conditions that can only be evaluated from server side code
use a master page for better and register the script block from the master page. This will make it easier to maintain if you keep the logic in one place
similar to the option above, you can use a base page class if using a master page is not possible
use an html script element if what you need is to reference a static js file
use place holders such as js variables and fetch dynamic data from the server using ajax
use unobtrusive javascript, custom data attributes and ajax
Whatever the suitable option(s) is depends on too many factor that you have to assess

Indexing ASYNC JavaScript content

Is there a technic existing to index AJAX content with google bot? I mean, is it possible to declare a method in javascript that the bot will call and execute to get page content?

No.
The sensible approach to dealing with this is to create real pages that express the state of the document is in after being manipulated with JavaScript, to link to them as normal and then progressively enhance with JavaScript to:
Cancel the normal action of the link
Modify the DOM
Change the URL in the address bar

How do I load an mbox content without inserting into the dom

I'm trying to interface with Adobe Test & Target because I want to load JSON rather than markup through my mbox. I want to load some mbox content into javascript and manually add it to the DOM. I have searched all over for full documentation of the mbox.js but I can't find anything other than the very basics. It describes how to use mboxDefine() and mboxUpdate to target a specific dom element. Is there a function that just returns the content?
```

T&T does not offer a function to assign the response to a javascript variable. Basically the way it works is mbox.js builds a url to their server and then then outputs a script include tag. This is done to get around the same origin policy limitations (cross-site scripting).
In order to handle whatever is in the html offer, they put it in their own javascript variable on their server and then output it as that as the response. However, they also have the response output the code that updates the target element. So there's nothing you can do to actually stop them from updating the target element with the html offer contents. They simply don't expose that.
However, you don't have to put html in an html offer. You can put json (javascript) in an html offer. Just do like
html offer 'myJsonMbox' (in interface)
<script type='text/javascript'>
var myJsonString = "[json string]";
</script>
Then on your page (inside your body tag, but before your code that wants to use it) you'd have the regular mbox code:
<div class='mboxDefault'></div>
<script type='test/javascript'>
mboxCreate('myJsonMbox');
</script>
And then somewhere after that, where you're wanting to do something with it, that myJsonString is there for you to reference. Or, you can do it with the mboxDefine and mboxUpdate sometime after page load, if you prefer.
Is there some particular reason why you don't think this will work for you?

You can:
a- Insert JS code you are going to use to manually manipulate the DOM
b- Insert CSS code you can use to alter the original HTMl or the newly added HTML.
c- Insert a call to a 3rd party script that will load content from a 3rd party server if needed, or the same server.

Possible to append a ActiveX control to a page using javascript?

I'm trying to append an ActiveX control dynamically to a page using jQuery. The append is successful; however, the control doesn't initialize when it is done this way. I believe IE calls the OnCreate method of an ActiveX control when a page that contains a control has finished rendering. The problem is that the tag is not present on the page until after rendering is finished, so OnCreate is never called.
I'm not sure if that's the problem, it's just a guess. Does anyone have experience with this? Is it possible to force IE to call OnCreate at a specific time?
The control works fine if the tag is in the html. The only time I see problems is when I add the object to the page via javascript.
Update: I need to know what IE does when it encounters an
<object>
tag on the page at render time. The control works fine in that context, so IE is calling something at that time. I need to invoke that manually after I've added the control to the page post render.
Thanks, Pete

You can instantiate the control in a totally cross-platform-unfriendly manner using new ActiveXObject(ProgID). ProgID is a string of the form "appName.typeName". e.g.,
var excel;
excel = new ActiveXObject("Excel.Application");
...
The example will only work if excel is installed on your machine.

I had a similar problem to yours today with a java applet under IE. My workaround (i wanted to put the applet after page has finished rendering) was to dynamically create invisible iframe with src pointing to simple html page with my applet. After loading iframe i called it's parent to notify that the applet was loaded.