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!