In terms of UCLA’s 110,000 applications, the UC admissions reading system is designed for scalability by having two readers score each application (with a third senior reader also reading it if the first two scores are too far apart). Then they are rank-ordered by the reading score within buckets defined by major or division. If the cut-point for the number of admits is within a group with the same score, there are designated tie-breaking procedures. The key difference between this method and that used in other holistic reading colleges is that there is no central admissions committee reading every application and therefore being the bottleneck when application volume gets huge. The process is scalable because they can just hire more admission readers to handle the greater application load in parallel.
In terms of being selectivity peers, both UCLA and USC are widely seen as selective enough that top end academic stats are needed to be competitive, but they are not as competitive as Stanford for those who do have top end academic stats (though UCLA engineering majors are more competitive than UCLA generally). But the nuance of HS GPA versus SAT/ACT scores is often not noticed, even though “unbalanced” applicants can be significantly affected.
Note that it was not always this way. USC used to be seen as a much less selective school before it made the push to climb the USNWR rankings (during that push to climb the rankings, it was very generous with National Merit Finalists – a great way to attract high-SAT students).