The busses make the compartments and passageways. There are formed reinforced cement walls and overhead (w/supporting pilings) that support the 5-14 ft of soil that provides the shielding against the most powerful radiation that might hit that area.

For example: 1 bus holds the Galley, 1 or 2 busses are joined so as to form a messdeck, 1 bus holds a command & control area, 1, 2, or 3 busses contain the female berthing areas and the same is true of the male berthing areas, 1 or 2 busses form classrooms for the children, etc, etc. It has been arranged very nicely & efficiently from the internal finished photos that I've seen.