I'm working on a mod and need help on where to get started on how to add ui design into the sims 4. For example, how could I add pop-up menu by pressing a button, and on that menu I'd like to add a meter bar. Which programs should I work with aside from Python and which files from python should I look at?
From what you explained and what I know, this doesn't sound like there's something similar already in the game to re-use, so I have to recommend not doing that. It requires knowing ActionScript, overriding common files (your mod will not work with Tmexi's main mods), and they break frequently. The alternative is to create a synched program like Lot51 is doing as it's safer. If you still want to look into it regardless though, you will need JPEXS, and I wrote a basic setup tutorial for extracting, etc.: https://www.patreon.com/posts/setup-for-ui-54708148
To register new UI/GFX files, I believe the only file you need is DE3FB8F4E5C1A9E5 - Font Configuration.
If you do happen to be aware of a mod that does something similar to what you want though, I would recommend going to ask the creator then because it means they are re-using EA's files (Tmexi is the only one who makes new UI files).
Creator Musings is a Sims 4 modder, poser/animator, and CC creator hangout server (though everyone is allowed) with a tutorial/resource directory, help channels, and mod/cc/sims 4 news channels! My Discord | Twitter | Tumblr | Patreon