Ok, so I've been thinking for a bit, and I know there's a ton of alternative "libre" frontends for websites like YouTube and Reddit. However, I was thinking: what If I just wanted to make my own frontend? Not specifically for the intentions of "libre", but I wanted to change the controls and appearence of the website. I'm honestly unsure of how to approach this. Any thoughts?
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.
I took a look at these, and decided that I'm gonna use something like flask as a lot of people seemed to express privacy concerns over all of them. If you disagree please let me know.
No offense but you should probably start by learning some frontend development (e.g. VueJs) because your pick sounds like you still don't understand the 'stack' architecture, divided between API endpoints (e.g. flask) and the reactive components (Javascript)…