You fire whatever weapons the unit inside is armed with. If you have a Platoon Command Squad in the back with four plasma guns, then you fire four plasma guns and whatever the LT has. Want to mount a Heavy Weapon squad in someone else's ride and let them use it as a bunker, then the three Heavy Weapon can all fire out the back hatch (assuming no movement, shaken, stunned). Forget the old hull-mounted lasguns exist, cause as far as the rules are concerned, they don't. You fire up to five weapons that the embarked unit is armed with from the large back hatch, subject to the vehicle movement or being shaken/stunned. That's all there is to it.