People interested in a "read the manual and code it up on real hardware"-type guide should take a look at Stanford's CS140E[1] repo! Students write a bare metal OS for a Raspberry Pi Zero W (ARMv6) in a series of labs, and we open source each year's code.
Disclaimer: I'm on the teaching team
[1]https://github.com/dddrrreee/cs140e-25win