“We don’t. We do know much less energy is required to damage the devices.”
Did you know that a very small number of vehicles tested by the EMP commission had any damage when subjected to a simulated worst-case pulse?
The vast majority had no impact, the majority of those impacted simply needed to be restarted. Others remained functional but lost significant systems. Some needed to be repaired.
That tells you a lot. If systems in a car can survive, many other systems will survive at statistically significant rates.
We know a great deal about the physics and it’s manifestation as electromagnetic energy. The biggest problem is the energy grid, and the willingness to invest in replacing and retrofitting things that aren’t broken.
I know there are a lot of sources which disagree with your statements.