I went to UCSD as an OOS student from the midwest many (many) moons ago, so my experience may or may not be relevant. I’ll share a bit of it so you can decide for yourself.
Everything in #2 above is correct. CA is large and students come from all over the state, they don’t know each other and often don’t even know where some of the towns in CA people are from are! My DH was forever clarifying exactly where his hometown was in relation to larger areas in the state. Californians are generally very friendly and open to outsiders. Like was said above, many people come here from elsewhere ( a common “get to know you question” as an adult at parties is “where are you from?”), so they are very interested in and welcoming to new people. With that in mind, as OOS student, expect to be asked “Why UCSD?”, a lot. For the rest of your life, seriously. I still get asked why when I tell people I came to CA from the midwest to go to undergrad there. Understand it’s not a malicious question, just a curious one. My stock answer was (and still is), the weather. The truth was more complex, but the weather is something everyone understands and usually leads to a nice laugh comparing the weather between SD and my home state.
Expect some degree of culture shock. Things will be different than back home, in ways that go beyond just - you’ve gone to college and are on your own - not necessarily bad different, or good different, but different. You need to be sure you are up to the adventure of that. I think it’s a great time of life to experience that, obviously, it was for me. But I also know every person is different. I had a roommate freshman year who was from Northern CA, had many HS friends at UCSD, and yet still felt very homesick and lonely. I did have a good experience at UCSD. I never felt isolated as an OOS student. I loved CA and ended up staying.
I wish you well wherever you decide to go!