Their new COSMIC desktop is generating a tonne of buzz. It may spill over to the distro in general.
I am not a PopOS user but, watching the evolution of COSMIC, System76 seems very user focussed and makes sensible decisions. That bodes well for the overall OS.
I cannot answer your question obviously but there are several “primary” distros.
Debian, Fedora, Arch, Void, Alpine, Chimera, RHEL, SUSE, Gentoo, and others are all built from scratch. You do not have to use SystemV. The closest to that is probably Slackware I guess.
PopOS is based on Ubuntu which is itself based on Debian.