1. Unless you define soil properties, and/or Site:GroundTemperature:XXX objects (eg Site:GroundTemperature:Surface) it'll assume a constant temperature of 18°C throughout the simulation for any surface in contact with the ground.
2. If your temporary building is sitting on a concrete slab, and it doesn't have another floor of its own, then that slab is its floor (conduction heat transfer goes from ground through that slab to the inside of the zone)
3. If this slab is uninsulated, varying the concrete thickness is almost irrelevant to be honest. Pick whatever, 20 or 30 cm will do.

TL;DR: use a BuildingSurface:Detailed with a Surface Type = Floor and an Outside Boundary Condition = Ground. Assign a Construction corresponding to 20cm concrete.

You can add a Site:GroundTemperature:Surface object to define the monthly temperature for the outside of this surface, and if you don't it'll use 18°C.