how can i copy audiobooks to a backup device? where are they on the mac?

I have a large number of audiobooks on my Mac, in Books, some from Apple but mostly Audible. They do not appear on my iPhone or iPad even though all are logged in to the same iCloud account. Why is this?

I want to copy them to a USB stick as backup but cannot find them in Finder. Books app does not seem to allow for copy/paste etc. Is this because they are proprietary items?

Unlike Music or Photos there isn't a Library in Finder for Books. Where do they live on the drive?

Any one can enlighten me please? Thanks