For graduate admissions, decisions are made centrally- ie the Faculty decides whether to offer a place, and how to allocate the funding they have access to. Your first choice college then sees the application and decides which people to accept at that college - if they don’t have room for you, you are still guaranteed a place, but your application then gets passed on to another college. College decisions can take ages at graduate level because there are so many moving parts, so you find out if you have a place quite fast but funding and college decisions can take several months longer.
Some graduate students get really into college life and others much less so. Academically your focus will be the Faculty, so the college is more a social hub (though you may choose to socialise via the faculty or extra curricular groups instead. It’s quite different to undergraduates whose college will be their first academic point of contact.