Case in point: AFAIK, some Raptor Computing machines have an ASpeed processor which is powered on first. It then loads the bootROM from a removable flash chip on the motherboard, the bootROM initializes the rest of the hardware and loads BMC from another flash chip on the motherboard. The BMC does whatever hardware initialization still must be done and only then it runs the bootloader. The bootloader (petitboot, I think) scan the drives for bootable media and shows a menu for the user to choose what to boot.

The ASpeed and the POWER computers are somewhat independent after that point. If I had enough money, I'd get one of those machines just for the peculiarities of its hardware.