Are you asking if you delete the Apple Books app from your iPhone if it will delete audiobooks purchased from Apple from your phone, I’m pretty sure the answer is yes. I don’t think these purchases show up in any other app.

If you delete them, you should be able to redownload them from the Books app/store without repurchasing. However, I would still make a backup of your purchases on a computer because record companies and other publishers have a nasty habit of pulling items out of circulation because of licensing disputes and whatnot. Better to have a local backup just in case.

https://support.apple.com/guide/books/download-purchased-and-updated-items-ibks888f2d84/mac

https://support.apple.com/guide/itunes/download-previous-purchases-itns36a8f323/windows