IIRC almost all Unixes followed the /src model you really only see in BSD today. Sometimes it was a separate license or only given to Universities and 3rd party developers. But before Linux it was assumed you could build the kernel and base userspace from there like you could with Bell Labs/AT&T.

The University/3rd party part was true even for Microsoft with NT and CE in the late 90s/early 00s. My little CS department got a CE dev kit with software, a device, and the source. I goofed around with it and was pretty quickly frustrated.

This also wanders into my memories of hating Sun version numbering and naming. SunOS grew SysV and became Solaris, but it was still SunOS. But it wasn’t. And then they preceded the Java-ism of the minor version being generally accepted as the major and conflictingly published materials both ways.

I feel bad for the devs where what they did made sense to them and then marketing took over. Naming and versioning are hard enough problems alone.