In a Recent build I flashed the wrong firmware to a f4 fc.. and after that I had a brand new tbs nano fail before I got to bind it.. and a day after my indoor hover test..I had no video (grey screen no osd).. changed the vtx no change..
I didn’t give it much thought, figured. I got hit by QC bug and chalked up to the game… but I remember, as I was trying to figure out the firmware issue.. I saw the voltage in betaflight (green batter) reading like 34-36v..
I know the large physical circuitry can’t change.. but can things be “re-directed” in the small circuitry enough to cause damage?? Like a micro voltage spike??
I’ve heard of fc being bricked, But the fc “responds” in betaflight as normal .. well on the screen..

It's unlikely IMO. You would only fry external devices if the voltage exceeded the maximum spec for the external device. The UART wires simply just get connected directly back to the MCU which just puts out 0V and 3.3V on those outputs. The voltage output level on the voltage pads of the FC aren't controlled by the firmware. Their voltage comes direct from hard-wired voltage regulators on the board.

The worst that can happen with an incorrectly flashed FC in my experience is that the resource mappings are incorrect which then just means it won't work properly.

My only plausible theory is that you had a short somewhere on the board (solder balls, solder splatters, whiskers of wire etc.) that caused an intermittent short circuit between voltage rails on the board which then allowed an out-of-range voltage to flow to the Nano RX and also the OSD chip on the FC resulting in them getting fried. Either that or a failed voltage regulator on the FC which then allowed the input pin voltage to flow to the output pin rather than a lower regulated voltage being output.