Easyest way, to begin with, is to use a browser addon and userscripts. Addons to look for are Greasemonkey, Tampermonkey, or Violentmonkey and alike alternatives. Keyword to google "userscripts". P.S. As I see it, from your description, you just want some mods (changes) to existing UI, no need to rebuild one from a scratch in other more complicated ways.

The generalized approach in industry is to use API calls, and create classes to structure the data you receive as JSON or XML. At that point, it is entirely up to you how to format and display the data from your classes. Take a look at some of the Lemmy client code like Mlem, Memmy, or Voyager as examples. Though they have gotten more complicated, they all follow this client-server model for front end development.

However, due to recent shenanigans around API and RSS by companies, mostly those looking to prevent AI companies from using their data for free, the alternative, much worse method is to take the HTML output from a standard web request, and try to reverse engineer the page information into a class structure. This sucks, breaks frequently, and requires you to code around ads and other junk on pages in order to get at the content.

A website is just textfiles that your browser downloads. You can open the debug menu and read them, and even edit them. Sometimes the files are minified or obfuscated and you need to use some special tools to make them readable. But the core concept is: the files are on your pc, you can do with them whatever you want.

If you don't know how to start, it seems to me that you wouldn't have enough skill to succeed. So instead I'd suggest you start with what's called "userscripting" (greasemonkey or similar). Userscripting is a way to modify a website on the fly. For example you could remove some elements, reposition elements, add buttons with new functionality etc. That will let you start with your goal on a small scale, possibly with some compromises from your vision. But you will be able to get into the general workflow and get results in a reasonable timeframe.

If you want to do it properly you have to study the API, which can be either publicly documented or you have to read the websites source to reverse engineer it. Then you just program a website like usual which uses the API and does something with the results. If there are already open source frontends, and no public API docs, you can read those projects source instead, which is easier than reverse engineering.

Hope this helps as an entry point

Scrape the website instead of using an api if the website offers it

I have no experience. Anyway:

• learn how to make a basic website in some flexible programming language like go, rust or python (django)
• make a list of functionality of your original website you want to provide an alternative frontend for (as a start, you can expand later)
• find a way to retrieve all important data for the most important functionality reliably
• implement it into your website
• make it fancy, expand functionality, use the same webpaths for you website like as the original (so it can easily be redirected with libredirect or similar)

You can also look at and analyze the source code of existing Libre frontends like rimgo (go).

if it were me I would use python with flask and requests to scrape the site and provide the data to alternate frontend. there might be more resource efficient ways to do it but this is what i know.