Kindle
How do I import books to my Kindle?
Select Account & Lists.
Click Your Content and Devices.
Click ... to the left of a book.
Click Download & transfer via USB.
Select your Kindle from the drop-down box.
Click OK.


Apple
How do I import audiobooks into Apple Books?
In the Books app on your Mac, choose File > Import. In the Open
dialog, select the books, audiobooks, or PDFs you want to import, then
click Import. Note: You can also drag and drop books, PDFs, and some
audiobooks into your library.