At a UC info night for our SF Bay Area school district, the college counselor (not a UC rep) told us that it’s very hard to transfer into a UC (and she was talking to in-state students) as a junior from a 4-yr college bc they hold those spaces for community college students on the Guaranteed Transfer Agreement (CC to UC/CSU).
I’m from the northeast (NJ) and have a friend from there who went to Cal for undergrad (ok, >30 yrs ago) and had no problem fitting in. I went to summer school there, and also had no problem making friends.