Commercial means to make money, not necessarily be efficient or environmentally friendly, or simple. Most of these places just grow greens. Green will grow on rafts just fine and need no substrate.

They discard the fish solids, or de-water them and compost them for use in dirt farming. Like others have said, if you want to be self sufficient, do it with gravel. You can raft too for greens, but you can't live on greens.