> 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.

That's not very different from depending on the BIOS/UEFI firmware on a PC; the main difference is that older Raspberry Pi didn't have a persistent flash ROM chip, and loaded its firmware from the SD card instead. Newer Raspberry Pi do have a persistent flash ROM chip, and no longer need to load the firmware from the SD card (you can still do it the old way for recovery).

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

Even this is not that unusual; AFAIK, in recent AMD x86 processors, a separate built-in ARM core takes control on power up, initializes the memory controller, and loads the initial code for the CPU. The unusual thing on Raspberry Pi is only that this separate "bootstrap core" is also used as the GPU.