If you're curious go walk through the bootloaders of an embedded platform, they serve roughly the same role as firmware that implements UEFI on traditional x86 systems.

Example, start with bl1 (first stage boot loader) for a Rasberry Pi here:

https://github.com/raspberrypi/arm-trusted-firmware

The Pi's hardware (register map, etc) is well documented so it's a good way to learn this stuff.