Just as with anything you buy, a book, a movie, music, it is on Apple’s servers, not yours. Unless you download it. If you buy a movie for example it is there to stream or download. If you don’t download it it still shows up in your library (Apple’s servers) not on your device or in your iCloud account. Same with music. Let’s say you buy the entire Beatles’ catalog. It shows as Purchased if you go to iTunes and search for ‘Beatles’. You could download the catalog and that would take up space on your device. But none of it is saved to your iCloud account. So it’s either on your device or on Apple’s servers. Or both. But not in your iCloud account. Kind of like iTunes Match. If that’s still a thing. For a small fee you could upload your music (that is CDs, vinyl records, anything you ripped) to Apple. They would match it, store it, and because it was your music, all inclusive, when you went to look at YOUR library everything (not just purchases) would be listed. This was not the case for movies, that I know of, but I’m trying to find out about books. As I mentioned Amazon Kindle lets you do books in that way.