Linux

Not my area, but since OSs are really low-level (obviously), they can be affected by details of the host architecture that we don't often think about. Endianness, for instance.

I opened up the source package for the kernel I'm currently running (6.1.42) and looked at it. The smallest set of architecture-specific code is the ~2MB for sh (I assume that's SuperH, a 32-bit RISC architecture from the early 1990s). 32-bit ARM takes up 27MB, although if you check the individual files, a fair amount of that is device trees and the like. So we're talking about less than 50MB of arch-specific source code for most platforms, and probably less than 10 in many cases, but it depends on the design of the architecture and how many times it's been extended.

Looking at individual file names, topics addressed in the kernel's arch-specific code files appear to include booting, low-level memory access, how to idle the CPU, crypto primitives, interrupts, suspending/hibernating the system and other power management, virtualization facilities if the CPU provides them, crash dumps and stack traces, and, yes, endianness.

You may also need additional drivers for odd bits of hardware not used by other systems. Or not, but it's a common sticking point with ARM SOCs and other small-format machines.

That's just the kernel. You'll also need to establish a working cross-compiler before you can get your kernel onto the system. At that point, you can probably bootstrap much of the rest by running make and get to a working command-line system (GUI is going to be more of a crapshoot, requiring additional work on video acceleration and such in order to run well). And there may be odd warts in other pieces of software, each requiring a few lines of code that add up over time.