I wouldn’t say the OS is Linux any more than the OS of an Apple computer is XNU. Linux is just the kernel. Similarly the other OS isn’t “Windows NT kernel,” but Windows 10 or Windows 11.

The kernel is linux. From wikipedia: An operating system (OS) is system software that manages computer hardware and software resources, and provides common services for computer programs.

Linux being designed to work with so many things (and so many things designed to work with linux) all combined make the OS. This is also where the famous quote comes from. https://www.gnu.org/gnu/incorrect-quotation.en.html

unironically: most distros are actually mostly gnu, just with a linux kernel. bit annoyed when people just call it “linux”