If you want the physics model for a vehicle, it's in the .Jbeam files of the vehicle. The physics model for maps and stuff is stored in the map files, and can be edited using the ingame world editor (press F11 to access it).
If you mean groundmodels, a list can be found here: \Steam\steamapps\common\BeamNG.drive\gameengine.zip\art\groundmodel.json Either use the name or one of the aliases (where available)
