Here we see the footings and frost walls of the new home installed to the level of the basement floor.
The frost walls are created with Insulated Concrete Forms (ICFs), which are foam blocks stacked like Legos and then filled with concrete. An excellent system.
Inside the frost wall perimeter, the area under the future slab is filled with sand. Sand is used because it compacts well and allows good drainage. The sand is placed in layers, with each layer compacted by a vibrating compactor machine. You can see the machine near the center of the picture.