for the fairphone 5 currently I’m only aware of /e/ OS. DivestOS and CalyxOS are in development but it’ll take a while.
I don’t think there is a “pure” linux distribution in development for the FP5. The FP4 has a somewhat working port of ubports (ubuntu touch) and sailfish OS.
The other OSes are open source. There is an ongoing german blog of a security professional who tests custom roms for privacy and security: kuketz blog custom roms. Check that if you want to know more about the details and differences, although some entries are old might already be adressed. There is also a post in this very forum reagarding alternativ OSes, I suggest checking that too: