…and Linux, Android, WebOS, FreeBSD, OpenBSD, NetBSD, DragonflyBSD, GNU Hurd, Minix, QNX, Inferno, L4, FreeDOS, OpenSolaris forks like Illumos, OpenIndina and SmartOS, the various embedded real time OSs, the various Sony PlayStation and Nintendo Switch system softwares, various different bootloaders and UEFI interfaces, networking firmware like Draytek firmware, Cisco IOS, etc.

And that’s not even covering the numerous hobby OSs out there like Haiku, SerenityOS, ReactOS, TempleOS, SkyOS.

Then you have experimental OSs like Singularity too. There’s numerous examples of them alone but I think you get my point. :)