It seems like a mixture of heuristics, explicit filtering and user reports.

https://help.kagi.com/kagi/features/slopstop.html

That's specifically for AI generated content, but there are other indicators like how many affiliate links are on the page and how many other users have downvoted the site in their results. The other aspect is network effect, in that everyone tunes their sites to rank highly on Google. That's presumably less effective on other indices?