I have no idea where they get their stuff.
I have noticed that their selection seems to be different then what one finds at mobilism & Z-library. Those two typically have the same set of books. I assume that people who visit one site finds something and posts it on the other.
On Zoboko, when you get to the page that is "converting" the ebook to the format you requested, the language is Polish, at least that's what google translate says.
Not sure why the book has to be "converted" regardless of the format you select.
The site is often wonky, as described in
this thread.