On a Mac, the Books app stores its audiobook files in a folder called "Audiobooks" here:
~/Library/Containers/com.apple.BKAgentService/Data/Documents/iBooks/Books
In the Audiobooks folder, subfolders with long names like "sha1-14bda2a1acec40a15657a306e3d12684f6ed3cbb" contain the individual mp3 files for each track of an audiobook imported from a CD or other external source.
To use Apple's iTunes replacement, the Music app, to import audiobooks to Apple's Books app:
1) Use the Music app to import the audio tracks from your audiobook CD's.
2) Use the Mac's Books app to File/Add to Library, point it at your iTunes library: /Macintosh HD/Users/[yourname]/Music/iTunes [your iOS device]/iTunes Media/Music
(Even though the iTunes app is no longer on your Mac now running Big Sur, the iTunes libraries are still there, and the Big Sur Music app will continue to use them to store imported files. Music will also create a new folder with a default name of "Music 1" and containing only a single file, "Music Library."