Thank you JBB!

Can you explain more about what you wrote:
"A common alternative is to also connect a reset and bootloadet entry pin. This way the Pi can reboot the micro into firmware update even if it’s crashed, and the firmware update code can be contained in a small bootloader that doesn’t change often. Many micros (eg STM32) have aerial bootloaders programmed in at the factory."
How does that work? If something doesn't go well during the updating of the firmware how does that pin/signal help exactly?
Could you briefly explain the entry code/logic at micro boot-up?
If the code messes up the overwriting of that very entry point would it not be irreversibly "locked/stuck"?
Also how would the programmer know that the burned code burned correctly into the micro?
Would that be a single pin going from the programmer (Pi) to the micro or also the same (or another one) communicating back to the programmer/Pi?
Thank you
