In my subject (History), we ask for up to 4 essays per student before interview, and these have to be ones the school have marked up (ie. the original handwritten comments and marks from the teacher). The school has to certify these are authentic. We also are v familiar with the A-level mark schemes and with the mark schemes for our own pre-interview tests.
If you read somewhere between 120 and 200 A-level essays each year for ten or twenty years, you get a pretty good sense of how they measure up, how much input (some) schools have, and what an essay by a really good candidate looks like compared to one who has been taught to hit the mark scheme. (Some of the most academic schools in the country can churn out some of the dullest essays which are nevertheless designed to get good marks at A-level.)
We also have the scores from pre-interview tests, GCSEs, A-level predictions, and the two interviews, in which there’ll normally be at least one unseen material test during the interview where candidates will be asked to read a piece just beforehand under exam conditions and then discuss it. There’s a huge amount of spreadsheet data on contextual and school background, comparisons to national and LEA exam scores, etc.
You quickly get a sense for the really impressive candidates, and the ones who would struggle with the degree. There is a band in the middle where you do have to carefully weigh up the pros and cons of different applicants and their backgrounds, and luckily the second college interview (Ox) or winter pool systems (Cam) really come into play here in terms of moderating applicants across the whole subject. Even so, there will still be applicants who could do all who don’t quite get in just because there aren’t enough places available. There’s traditionally quite a high rate of success on second applications the following year.
It’s definitely a bit of an art as well as a science and requires a judicious and knowledgeable eye; but everyone takes it super seriously, and there are normally several academics interviewing and assessing each subject across a range of criteria. In my experience, it’s heavily professionalised these days; and the decision making is very detailed as everyone involved is accountable for the student if they do get admitted! There are no trick questions or rogue decisions. Every applicant you offer to will have been scrutinised several times over by Admissions and several other tutors.