My guess is part of it’s to do with a much higher proportion of students at top institutions (than at sub-top-1%) having attended prep schools that teach in small group discussion-based seminars rather than traditional classes. Changes how you converse, changes what you consider normal for interacting with your instructors and peers, carries over to the cultures that develop at universities they attend. Plus just everyone there being really driven to get good grades.
I also noticed watching online courses from top universities in a couple areas I’m fairly familiar with from having taken similar courses at a maybe-3rd-tier university, that the content and quality of the lectures was basically the same—the difference was entirely in how engaged the students were, and the kinds of guest lecturers they can pull in for a visit (“holy shit, I know that guy!”)