What is actually happening when you hard brick a phone?
It has nothing to do with the processor, unless you've fried it.

Usually, the only software-based hard brick that is possible is when you overwrite the bootloader.  Without a (working) bootloader, the phone (a) cannot get to the point where it would load the operating system and/or (b) cannot have working software loaded onto it.

If you get into this state you need to access the hardware more directly to write a new bootloader, often using JTAG.