the usual CS textbooky approach would be to start with something like Minix, or some small microkernels such as seL4
other "mini operating systems" are bootloaders like GRUB and u-boot

then combine it with some more general Operating Systems book that explains the fundamentals (i know no recent ones, sorry, that's too long ago for me)

but don't be too intimidated by Linux, it's not all very complex code, and it's fairly well documented, though there *is* a lot of it 

 Very well, I shall take this into account. My aim here isn’t to build anything specific yet. It’s more about learning. The textbooky approach seems good enough for me. Thank you! 

 Okay, let me do some searching and digging and Ill see what else I have saved that might help!

Have you looked into FreeRTOs? I had to dabble the tiniest bit when working on a school project a while ago. Then I got into directly working with Atmel hardware bare metal. From 2018-2023 I spent learning early 8065 (pre x86 but tightly coupled to early stuff)

Something also worth looking into is the Cosmo LibC project https://github.com/jart/cosmopolitan. Justine is awesome and so is her discord! I mention this because most of her library is highly hardware optimized and plenty of asm to go around. 

There are a couple other folks in the discord like ahgamut and paulclinger along with some other peeps from NIST and other orgs that are just awesome to read and chat with too 

 You just unlocked a memory. I’ve worked with FreeRTOS on Atmega chips for competitions and school projects as well, but that was such a long time ago. 

 I just check this repo’s readme, I love the idea. I’m gonna follow them. Thanks for the suggestion. 

 NP! The ELF stuff is way over my head, but they have done some pretty wicked OS "hacking"  

 Everything is “over our heads” before we get into it ! 

 t-y Chip Turner