You need a compatible phone depending on the distro. There are Mobian, PostmarketOS, Ubuntu Touch and ArchLinux Arm with Danct12 repos and others.
Right now the most supported phones are PinePhone and PinePhone Pro. Under "supported" I mean hardware support.
I have both. GNU/Linux on phones is not as polished and convenient as Android, but usable. I daily drive Pro model.
PineTime is nice, wearing it right now.