[apparent]

Everyone knows that UC still practices AA, though not named as such. And UCSD doesn't just admit students who didn't get into UCLA and UCB. Students who get into the latter also get into UCSD. Some much lower-ranked UCs might not admit these students because they assume they'll not come (yield managing), but UCSD is close enough that some students might choose it, especially if they want to be near on the beach/in SD.

[aprilthird2021]

No, everyone does not know that and it's not true. UC have a much different demographic skew than private California schools which do practice AA (e.g. compare Berkeley and Stanford)