This is the first carpentry step in framing the house. Floor deck goes on top of the foundation, and they you are ready to start on the walls. Then it will really start to look like a Plat House!
The builder is using truss joists, or I-Joists, which is good. The Plat House has an 18ft span for the main floor which is pushing it for regular lumber. The I-Joists can be made stiffer, and the floor less bouncy.