Interesting, I was not aware! Do you have a statistics for the bit flips in RAM %? My feeling would be its the majority of bit flips that happen, but I can be wrong.
It would be quite hard to gather that data and would be highly dependent on hardware and source of bit flip.
But there's volatile and nonvolatile memory all over in a computer and anywhere data is in flight be it inside the CPU or in any wires, traces, or other chips along the data path can be subject to interference, cosmic rays, heat or voltage related errors, etc.
It should be fairly easy to see statistically if ECC helps, people do run Firefox on it.
The number of bits in registers, busses, cache layers is very small compared to the number in RAM. Obviously they might be hotter or more likely to flip.
That was in the 2000s though, and for embedded memory above 65nm. I would expect smaller sizes to be more error-prone.