Z-Library was essentially a clone of LibGen with a more accessible UX and monetization. Though I believe they eventually solicited new material that was not fed back into LibGen.

Anna had a post about this. Basically libgen has more strict requirements on metadata, fiction/non fiction split, so it makes meeting difficult.

Most of it probably is on libgen. Putting all the eggs into one basket doesn't seem like a good move when booksharing sites keep getting shut down.

Not according to the author:

We are happy to announce that we have gotten all books that were added to the Z-Library between our last mirror and August 2022. We have also gone back and scraped some books that we missed the first time around. All in all, this new collection is about 24TB, which is much bigger than the last one (7TB). Our mirror is now 31TB in total. Again, we deduplicated against Library Genesis, since there are already torrents available for that collection.

http://annas-blog.org/blog-3x-new-books.html

From what I've heard, about 5% of it isn't on Libgen. Assuming that's accurate, that's still several hundred thousand books and several million articles.