How would boot block recovery work without a CPU installed at all? Because modern bios flashback does indeed read from the USB and write to one of the bios chi

How would boot block recovery work without a CPU installed at all? Because modern bios flashback does indeed read from the USB and write to one of the bios chips without even the CPU in place. Obviously there is some form of micro-controller performing this. On intel, I suppose it would be possible for the management engine CPU in the chipset to do this, but I doubt intel lets motherboard makers run custom code on that, so if it is not some standardized feature, it could not be that. On AMD Ryzen, I'm not aware of a CPU in the chipset. This all makes me think there is some other microcontroller somewhere that controls the bios flashback process, which would almost certainly be an extra chip.