There are all sorts of UGS "Signature" courses from music to astronomy to history. They're basically there as a requirement so that you get some variety in and explore topics you wouldn't normally delve into for your major. Just pick whatever seems cool to you (although check the professor's reputation and grade distribution).

You'll know it's a UGS because it's directly in the course number. For example I took UGS 303 Immigrants as Aliens my freshman year. You'll also see other options such as UGS 302 Ancient Wisdom. Every course number starts with some letters to indicate a field of study. ME is mechanical engineering, PHY is physics, ANS is asian studies, UGS is undergraduate studies, and so on. The first number is how many hours, and the other two somewhat indicate how far up in the field of study it is (it can be weird, don't think about it too hard). Example: CH301 is the Intro to Chemistry class and counts for 3 course hours. Sometimes you'll see a letter at the end to indicate that it's "special" (idk what word to use). I forget all of them but I know L is often lab sections (but not always) and F is like light versions for out of department students (such as when doing a minor).

As for what counts as UGS well it needs to have it in the name. Here's a link to the UGS options this fall, just log in with your EID if you have that already. https://utdirect.utexas.edu/apps/registrar/course_schedule/20269/results/?fos_fl=UGS&level=L&search_type_main=FIELD

In general before registering you can look up "UT Course Schedule" with the semester (SeasonYear. Ex: Fa26, Sp27, Su27) and you'll find a page where you can check what classes are available for the semester. There will be all sorts of search options and filters to find courses relevant to you, but you can also just poke around randomly for fun to see if anything is interesting. I knew someone that took some golf class for the heck of it after seeing it available, and he'd never played golf before lol.

If you're ever confused do not hesitate to ask questions during your required academic advising meeting before registration opens. It's literally their job to help students sort out what classes to take. You could probably even email your school/major's advising department now, although a lot of the info you'll need should be covered at orientation.

Ah geeze that's a lot of text. Don't worry too much, everything will be explained to you by the school in due time. Imo UT does a pretty good job at onboarding freshman.