The fire box is on the back side of the grill ( on all of the grills ) it is the entire length of the grill .  at least .250 wall ( quarter inch ) . all the grills are built on basically the same design ,just different sizes
The damper holes in the bottom let you control your temperture and also serve as the clean out . you can wash your trays and wash the belly of the grill back into the fire box and then out through the damper holes ,with the adjustable slide on the fire box and the dampers on the exhaust pipes you can set your temperture fairly easy and hold it at what temperture you desire