I found this on a recent inspection of a true prefab manufactured home, not a modular. There are a couple of dealers in my market that sell these. The sections are factory constructed and are equivalent to a stick built home, except the sections are small enough to easily transport and place with a crane. The foundation walls are precast concrete sections, with 1.5" Dow Board over the exterior, and I assume field cast in place concrete poured against the outside of the insulation board, however this exterior concrete maybe part of the panel. I did not see any joints at the visible foundation wall, but forming at this location would seem to negate the advantages of using a precast system. There are three photos. One of the overview of the system from inside, one at a typical vertical joint, and one at the bottom rail of the system at a joint, showing the connection detail.