Z-Library is a superset of Libgen, if I remember correctly it has lots of files that haven't been cleaned up enough to put into Libgen.

Z-library began as a Libgen mirror, but stopped mirroring uploads back. In some cases, it has documents that Libgen does not.

z-library uses a freemium model, a for-profit model.

Last time I tried, they prevented me from downloading a 6th ebook unless I paid. That’s a bit more aggressive than “accepting donations”.

I don't defend them pushing for payment but did you not also have the options of A: waiting 24hrs then downloading it. B: registering an account to get 20 downloads per day ?

I saw that today (and created an account); I don’t remember it was like that when I used it last time but I could well be wrong, it was a couple of years ago. Sure, you could wait 24 hours or get a new IP address and clear cookies to bypass the limitation altogether. I cannot say how much money they did with this, but that’s the definition of freemium.

freemium is a for profit model, the model has nothing to do if they make a profit.