what is uni/social life like in America for under 21s?

I did the same except I was of age and everyone had fake IDs for bars they've had since high school, they check IDs but not as thoroughly or specifically as we do in the UK, plenty of dorm/house parties compared to the UK I found there were more campus events and no one really went out to bars/clubs since most were under 21. Most times I did go out to bars and clubs, it was with older friends I had made in my apartment complex (23-25 ish) since I didn't live in dorms but a regular apartment. Then only went to bars/clubs a couple times with uni friends who were turning 21 or over 21, and with younger students it was almost always house parties/dorm parties. Usually older students will buy alcohol for you/for parties etc or there will be alcohol or a shop that is known to not really ID etc. It's the last of your worries and just have fun! You'll likely have more opportunities to drink than you think. Since it's still CV19 times it'll be happening much less anyway. Unlike the UK, at least with the American university I went to, people adhere to social distancing rules in the dorms and take it quite seriously I've seen. Where as my old halls here in the UK (I don't live in anymore but of course seeing first/second years and third years who still do) they've been ignorant of the rules etc.