There’s some variety in how library digital book systems work.
Cloud Library: we have to buy every copy of each book on these. So, if all copies are checked out, you place a hold. Of course we wish that you did not have to wait, but we’d need an unlimited budget to accomplish zero holds lists. Digital library audiobooks can cost as much as $109 per copy. Library ebooks can cost as much as $65 per copy, and many of the "cheaper" ebook copies ($12-60) actually *expire* after a certain number of checkouts or a time period (like a year).
Enki Library: also has specific copies, but as the collection there is quite large holds lists are uncommon (or very short).
No Holds Systems: the library doesn’t buy individual books. These work more like a rental system - the library pays based on how many books are checked out (Freading). Mostly it is medium-small publishers willing to do this type. The biggest publishers would much rather have us buy actual copies.