Operating systems primarily tailored for embedded systems. Like the “conventional” OSes, a large variety of architectures can be featured here.
Another excellent guide for getting the ropes of Embedded Linux.
https://www.thirtythreeforty.net/posts/2019/08/mastering-embedded-linux-part-1-concepts/
A long and very informative writeup about Embedded Linux, primarily but not necessarily focused on ARM.
An OS for various little microcontrollers and boards, originating from Acorn and their development of the ARM platform.
A bunch of links to 90s era Linux embedded OS projects, from linuxjournal.
https://www.linuxjournal.com/files/linuxjournal.com/linuxjournal/articles/039/3980/3980s1.html
A project aiming to put Linux on many embedded systems, without being restricted by the architecture of said system.