That corroded nail plate contacted the copper domestic water supply pipe or pipe sweat corrosion. Take your pick. Galvanic reaction. Condensate corrosion.
No flooding stains on the CMU. Sand looks clean. No barrier.
Looks like a GC went and did their own updates.
I can not see an SE OK the plywood sistering on the floor trusses or the fastening type for that matter.
1: Request disclosure from the vendor.
2: Recommend further review from a GC with in-house SE. Cheep by the way.
Those corroded truss plates are a potentially very serious issue. Recommend a structural engineer pull off plywood in a number of representative places to get a clear idea of how seriously the trusses are structurally compromised overall. I have performed engineer-specified repairs to compromised trusses and the corrections included plywood that was glued and specified a whole lot more fasteners than your photos show.