Manuals for the system and it’s processor exist. The Linux kernel is pretty agnostic towards what it’s running on, as long as it supports the C Language. The difficult part is filling in the system specific blanks in the kernels’ source code, which is big work.
Simple ports to small systems are actually very easy. To run Linux all that you strictly need is a timer driver and an interrupt controller driver, assuming the CPU architecture is already supported and there is a compatible MMU. On small systems, those drivers can be very simple.
I was bored one evening and ported Linux to run on the Wii's IO/security processor (not the main CPU, that's Wii Linux). It only took a few hours to get the kernel booting to userspace.
