I don't know whether you can now change, but if you could I suspect they'd give you the same options s everyone else, which is to pick one preferred choice of college.
You live in your college during first year (sometimes other years too, if you want and can). You're part of a JCR which is basically all of th undergraduates at that college. Together you elect people who represent yoh, like a welfare officer and sports officer.
College organises events like balls and formals and bar crawls. There are societies and sports teams.
So there will be one (sometimes multiple) site(s) which forms the college. Butler, for example, has three blocks of accommodation, a building with bar/JCR office and lounge, porters, etc, as well as a building with a sports hall, laundry, gym etc.
Nothing academic happens in college really, you can do any subject at any college, no teaching happens there.
If it helps to reassure you, my experience is that the most modern of colleges retain a strong identity and traditions, you don't miss out at all. I'm at Butler, we have gowned formals, we have chants, we have traditions.