According to https://www.universityofcalifornia.edu/infocenter/freshman-admissions-summary , first-generation-to-college students made up 32% of UCLA applicants in 2018, but 20% of UCLA admits and 27% of UCLA enrollees that year. (So much for the supposed 30% reservation for them in frosh admissions.)
Admit rate was 17% for non-first-generation-to-college and 9% for first-generation-to-college. But yield was higher for first-generation-to-college admits at 52% versus 36% for non-first-generation-to-college admits.
Note that first-generation-to-college students make up a much higher percentage of transfer students than frosh. In 2016, first-generation-to-college students made up 45% of transfer students, versus 28% of frosh.
Remember that about 68% of families in California are headed by parents without bachelor’s degrees, so first-generation-to-college students come from an underrepresented demographic by all of the above measures.
Regarding the immigrants, there have been a lot of highly educated immigrants to California over the past few decades. Their kids (in addition to the kids of US-born college educated people in California) are likely many of the ones adding to the competition for admission to UCs, regardless of what the kids of non-college-educated parents are doing with respect to college applications.