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