Libby sources most of their book files from the Kindle library of ebook files. That’s why most of them work only if you send them to Kindle. There are a few that aren’t, but most are. That means that your library is paying Libby, usually $5-10 for 1 copy to be borrowed 5-10 times before it has to be “bought” again, then Libby is paying some of that money to Amazon for the files, aka licensing digital copies to be loaned out. Libby is just a middleman charging the libraries for lending copies…