In my usage at least, Calibre is an eBook library-manager, editor, and conversion tool... which has a so-so eBook reader attached that I mostly forget about. :D
It's pretty essential for getting things onto the devices that I actually use to read, though.
Foliate is a much better reader IMO, but Foliate cannot do everything Calibre can. Calibre, on the other hand, can do everything Foliate does.
Calibre is much more than just an eBook Reader. Foliate is a very good (the best for computers, maybe? I don't know how it pales agains Apple Books) eBook reader.