Welcome to /c/piracy
No netflix or streaming services landlubbers allowed, this is pirates territory.
Certain forums place bounties on a book, and when someone uploads it they collect that bounty. It's a sort of internal points system for that forum. So someone will put up a bounty post, other interested users will also pledge, someone will obtain and upload the book to a file hosting site, and collect the bounty.
Some titles at least one person will have to buy. Other don't even cost anything, as it is trivial to rip a Kindle Unlimited book if you know what you're doing.
That could be part of it but I'm not sure it comes close for most ebooks. A lot of these books are professionally typeset and laid out pdfs, not sloppy calibre conversion pdfs. I don't think Amazon sells in pdf. As far as bounty forums, I just looked at one site over the last three days. It averaged 365 books per day, 199 per day with 2023 publication dates. That seems like a lot of successful bounties!