Surely the question seems stupid, but I'm doing a study on data recovery.

Assuming a hard drive with the broken electronic part (PCB) and without the cover where the model appears (we do not know the model and therefore we do not know which Firmware it has)

Could the platters be transplanted and read? Or a platter, even having been rebuilt by MFM (yes, I know, this technique is almost impossible and it would take years) can not be read without its corresponding firmware?

Thank you.

I would say the chances are good that a data recovery specialist can recognize the drive by looking at it, and deduce the required firmware.
It is a usual practice to backup the firmware of every drive that gets recovered, to have firmware on stock for cases where the firmware is missing or corrupted.
So yes, the firmware is essential, but it should be most likely doable to identify the required firmware.

What if the pcb needs to change the bios / firmware to another board? I mean if need to swap the original bios from pcb to the other pcb. For example, Toshiba PCBs need the original BIOS to work.