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> but I wouldn’t have the slightest idea where to begin

https://wiki.osdev.org/Expanded_Main_Page

It's in some way easier today than in the past. VMs really make testing much easier. On the other hand, the hardware has become more complex compared to some years ago when you could assume that all you needed was a little bit of assembly code to initialize the 32-bit mode of the x86 CPU and talk to the IDE interface.

Design and build? Yes, very much so. It'll be simple, but it's doable. How useful it becomes is a function of how much time you spend on it, and how many others you get interested in it.

Lol - I found a wikihow page to answer your question. :-D

https://www.wikihow.com/Make-a-Computer-Operating-System

Yes, depending on how basic, or how long you want to work on it. Gary Kildall did CP-M. Tim Patterson did 86-DOS (later bought by Microsoft and turned into MS-DOS). Terry Davis did TempleOS. And I'm sure there have been other, one-off operating systems written by single programers.