I've never used plex so this might be completely out of the ordinary for even more normal plex use cases, but is there a web client?
I use Android, iPadOS, Fedora Linux, MacOS and possibly in the future Windows (my last job required it, my next job might too) and unless the standard plex app supports audiobooks I doubt I'll find much consistency using native apps for each of those operating systems, if they even exist on all of them
Yes, they have a web client, but their audiobook support is not great. Plex is primarily geared towards music (albums/tracks), and the podcast support is built on top of it by independent clients. I recommend Prologue for iOS and Chronicle for Android.
I've heard good things about audiobookshelf [1] as a cross-platform alternative, but it is pretty early in development.
I use Android, iPadOS, Fedora Linux, MacOS and possibly in the future Windows (my last job required it, my next job might too) and unless the standard plex app supports audiobooks I doubt I'll find much consistency using native apps for each of those operating systems, if they even exist on all of them