interesting operating systems with quite some differing approaches:

OS/ Eumel
9P: network filesystem protocol, Plan 9, Styx, expose everything as simple virtual file systems
https://en.wikipedia.org/wiki/Inferno_(operating_system)

https://littleosbook.github.io/
https://klacansky.com/notes/useless-operating-system.html

https://github.com/cirosantilli/x86-bare-metal-examples