Actually, the fiery explosions come from the fuel and oxygen inside the pressurized ship. Now, that doesn’t mean the way it occurs in the films is completely realistic.
I guess you could say the plasma would have something to do with it too but I am lead to believe that the vacuum of space would negate any sort of reaction. Also speaking of plasma, any advanced civilization would know that heavy objects propelled at high speed would be more effective than any laser. But blaster bolts and what not look cool.