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.

https://jaycarlson.net/embedded-linux/

An OS for various little microcontrollers and boards, originating from Acorn and their development of the ARM platform.

https://www.riscosopen.org/content/

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.

https://www.yoctoproject.org/