After the slab was poured, the guys framed the walls on the lower level. Like most framing, this happened in a snap, maybe two days for two guys. They lay out the walls with a chalk line and crayon on the slab and then frame up the walls.
The only slightly tricky thing was deciding how to insulate. The foundation is rubble stone and while pretty dry, it is prone to weeping once or twice a year. We did install a french drain on the perimeter, just in case, leading to a sump in the mechanical room. That’s been bone dry for five years now. We decided to leave a 1 inch gap between the rubble stone and the walls for ventilation and drainage. The guys did this by framing the walls with conventional 2×4s and then gluing 2″ rigid foam panels to the back of the wall (while still on the floor), taping the seams, and tilting the wall up. They glued the wall down to the slab, fastened the anchor bolts where they existed, and then nailed the top plate of the wall up against the floor joists above. These walls are non-bearing, but they are really solid and well insulated. I had the guys stuff the joist bays above the walls with fiberglass batts as well.
Those I-beams on the floor are for the double-i-beam system I described in the post on steel and stone.
Incidentally, as I write this post, I’m sitting at my desk right against that middle wall. The finished room is shown below.