> A mostly fixed set of hardware

But it's not. Over time they've revised the SOC (processor) and gone from 32 to 64 bit capability. The latest - Pi 5 - has totally re-architected the I/O subsystem, putting most I/O functions on their RP1 chip and connecting that to the SOC using PCIE.

And as already mentioned, the unusual boot sequence: The GPU takes control on power up and loads the initial code for the CPU.

All of the OSs I'm aware of that run on the Pi depend on "firmware" from the Raspberry Pi folk. Looking at the files in the folder that holds this stuff, it's pretty clear that every variant of the Pi has a file that somehow characterizes it.