My “I’m a CS” colleagues all tell stories about the OS they created in uni. A couple of them were happy stories.
https://ocw.mit.edu/courses/6-828-operating-system-engineering-fall-2012/
It’s good to know how to do it, but there is not much need to go and do it.
It was a thing - https://en.wikipedia.org/wiki/Doping_in_East_Germany