Library Genesis has an "Upload" button on the top bar, the common login and password for which can be found from simple a Google search. They might also run more organised scanning/sourcing operations on their forums.

> Where do these books come from in the first place? Have publishers systems been leaked/hacked?

Most of zlib is libgen, and I think libgen relies on user uploads and sourcing from their forums

At least in torrenting communities a lot of these books come from book databases like OverDrive. Most public libraries give access to OverDrive and removing DRM is straightforward

In the first place, the books come authors who write words based on what a muse or ChatGPT tells them.