@GeonBit Hey, thanks for the reply, and yeah I edited my OP for clarity. I meant the coding itself, as I already know how to design UIs art-wise, plus there's tons of videos on that online anyways. Just wanted to know about the code.
But I see you provided some nice UI libs as well, I'll definitely look into those and read some documentation etc. Thanks.
By SetData I was referring to setting a Color and transparency for a given texture and rendering that texture on-screen as a line, rectangle, circle or whatever shape (as to hardcode some parts rather using existing image files, when I'm just gonna render a single-colored UI element anyways). Heck, I may even just end up doing no art assets at all (except typefaces) for my UI, similar to what you see in Factorio and similar games.
I've looked into 2D Primitives APIs though, but I'd like to just know how to do it myself for minimum third-party code and also for personal learning reasons.