The Common App is great, but it's not magically less open to bias than standardized test scores.
LOL. My understanding is the really good prep school college counsellors golf with one or more high-up folks in elite university admissions offices, and get the inside scoop on exactly what they and their peers in other universities are looking for in any given year, such that they can even tune an essay or letter of reference for a given school based on that non-public information and advise students which schools to focus their application efforts on, based on their background & activities.
Many schools, selective or not, actually do this whole process — multiple times, with each admissions agent doing a separate order of criteria, to ensure everyone’s application gets read at least twice. The idea being that those with the most “let’s give them another look” across the board are the most notable. Then from that shortlist the debates comparing each applicant, usually sorted by geographic proximity to each other, begin (at Harvard, if you’re from Texas you’re not really competing against New Yorkers for a spot, you’re competing against other Texans for the XX number of Texan spots they usually admit a year).
I did a short stint as a student worker in the admissions office of a very selective college in California (<5% admission rate, but not one many could name off the top of their head), and this is more or less how it worked