I’m new to this too and haven’t even started building my system yet, but from what I’ve read on the site so far it looks like they mostly use clay pellets for the roots to grow on. The plants don’t really grow in a soil. I think the point is to give the roots access to air, liquid nutrient from the watering and making sure the roots are protected from light so alge doesn’t grow on them.

Like I said before tho, I’m absolutely no expert. I’ve only gleaned this from reading on this site.