The volts serve to create the arc (though for lightning it's a bit more complicated than that), what matters in your car is the current that flows through the body. For typical lightning strikes here that's 5000-10000 Amps. That sounds high, but domestic mains is designed for a fault current level (think short-circuit) of 3000A. OHLE is now 6000A, I think.
There's a second stage, though. That current, flowing via the resistance of the car body (or whatever else lies between it and earth, or any two points within reach) creates a voltage (touch potential) and that needs to be low enough not to harm you (say 50V). It's not an exact science though.
Steel body shells ought to be OK, and but if you reach up to the top of a pillar and hold something else lower down you might be in trouble with a big arc/strike. But that's not very easy, as the surfaces are mostly insulating, at lowish voltages.
Old-fashioned fibreglass is, of course, no use at all. Carbon fibres do conduct, but over long distances only lengthwise. Composites vary a lot, depending on how the fibres are laid up. Making connections to them is tricky (even making sound fixing hard-points takes care). Aircraft composites have metal foils and coating applied, and go through some pretty thorough testing. they don't always survive entirely unscathed, but damage is superficial and not a threat. Car makers could exploit that knowledge, but I'm not sure if even Elon Musk has.
