The default is Fairphone OS. It is Google Android but they don't pay to license the name 'Android', so they have to use an alternative name.
You can install /e/OS (even preinstalled by Murena) or DivestOS. Any other option doesn't make sense as Divest is best in terms of security and privacy among all available custom roms and /e/ is most convenient (good but not perfect privacy, bad security but with MicroG, relock and App Lounge as well as Murena Cloud)