My three oldest children slid off the road in an ice storm, and managed to puncture the fuel tank on my GM product -- I had to drive it 60 miles dripping gas through a plug made from ice and leave it dripping gas in the dealers lot -- Sad thing is it never would have happend on my little toyota truck, as that had a full set of metal skid-plates for the underbody -- It really just varies from model to model and there are no standards for protection from this type of thing that I know of - It is up to each vehicles design team or the owner to buy aftermarket add on's to protect the vehicle underside.
I've plugged a hole in a gas tank with mud (not really a good idea) and a hole in an oil pan with soap (got me about 50 miles).
I've never had a car fire myself, but I've seen the aftermath of a few. In every case that I've seen, the passenger compartment did not look survivable. In both of the Tesla cases, my understanding is that the fire did not penetrate either passenger compartment. Obviously there are a lot of factors involved, but based on a very small sampling, I'm pretty impressed.