Particularly for electric cars - you have very inflamable lithium batteris, which when spiked very hard (by something sharp), so multiple layers short circuit and well short circuits leads to fire. But batteries are at the bottom of the body, with few easily exposed areas (otherwise they would be at risk of explosions all time). I do not know what people are doing exactly, but you can use a crowbar like thing to hit the beneath posterior, until you expose the battery, and then 1 or 2 hits can do it, or you can skip this and use a smaller conventional explosive.

I would like to add this - I do not condone arson. Even if anyone wants to protest - this is not the way to do it, unless you burn your own stuff, then you are only harming environment. What i wrote is purely for educational purposes.