For students who are planning on transferring to a UC for these majors, which would be the best? Out of all 9 UCs(excluding UCSF), which one would you recommend for these majors? I understand that one has to get a feel of the environment by making an appearance on campus first. Would they mind if you entered their lecture halls and listened to their lectures without being a student there?
For a large class, no one will notice if you just sit in on a lecture.
“STEM majors” is a rather broad group of majors, although UCs are generally good for those that they do offer (not every UC offers every major, though). But some may be different to gain admission to the major even after enrolling at the campus (e.g. CS at many UCs).