Depending on the platform, sometimes you may get audiobooks in MP3 format. However, MP3 is not the best format for audiobooks. It doesn't support chapters, and it's not bookmarkable. M4B is the best format for audiobooks. It supports chapters and is bookmarkable. It's also the format used by Apple for audiobooks.
Last year, I discovered the only MacOS that simply convert multiple MP3 files into one M4B file. It's called AudioBookBinder App Store page and can be found on the App Store for free!
AudioBook Binder is a simple app that does one thing and does it well. It converts MP3 files into M4B files. You drag your MP3 files into the app, and the cover, and it will convert them into one M4B file.
The options are limited but sufficient. You can set the cover, the title, the author, and the genre. I usually leave it to "Audiobook" for the genre.
I hope you find this app useful as I did.
And in case you store some audiobooks on your own server, you take a look at audiobookshelf. It's a Google and Apple app that allows you to listen to your audiobooks from your own server. It's open-source and free. You're welcome!
sponsoring me on Github or perhaps buy me a cup of coffee 😊