They seem to be using Molotov cocktails - that is, about a liter of gasoline ignited and spread when the bottle breaks. Since the car body itself is metal and glass, I would guess that until the battery ignites, it’s much the same mechanism of any other car burning.

Plastics in the wheel wells, mirrors, tires are ignited, which burn hot enough to ignite more protected plastics. Eventually, the battery is heated to the point of thermal runaway (analogous to the fuel tank in an internal combustion car), and then it burns to the ground.