UC does not use or even see class rank as given on the high school transcript.
If you are from a California high school, UC does compare your UC-recalculated weighted-capped GPA with a threshold GPA for your high school. This threshold GPA is from a recent past class for which the high school submitted grade records to UC for. UC-recalculated weighted-capped GPAs from the grade records determine the top 9% threshold. If your UC-recalculated weighted-capped GPA exceeds the threshold, you are “eligible in local context”, which typically means that if you do not get into any UC that you apply to, you may be offered admission to a UC where there is space available (typically UC Merced). ELC is also noted in your application and may influence admission readers.
In general, UC admission reading does not explicitly compare students from the same high school; admission readers give a single score after reading (two admission readers per application; if their scores differ by too much, a senior reader also reads it).
However, after applications are ranked/batched by reading scores for final determination of admission results, applicants at the margins may be compared against others from the same high schools to get to a consistent admission result within a high school. I.e. an applicant at the margin may move from deny to spring admission or spring admission to fall admission if academically weaker applicants were admitted from the same high school. (see Hout report)