With the support piers poured, the next phase is to do the concrete forms and inserting some materials that deals with expansion and compression of the soil. The plywood is to hold the concrete when it is poured.
From the first picture, the layout of the house becomes clearer.
This is the picture of what’s between the plywood:
At the bottom is the cardboard like things that will deal with the expansion/compression. Once the concrete is poured, we’ll never see those things again even though they cost me an arm and a leg…