The roots grow into the spaces between the pebbles.
You can fill the tray for a 'bed', you can put the plants in baskets and then insert them into the bed, or you can run modular (this is what I personally recommend, ESPECIALLY from seed!!!) by filling quart pots with pebbles, floating the pebbles by dunking the pot in a big container of water, and gently sliding the rooted clone in there- or if it's from seed, you can just put a rapidrooter plug in.
I have a couple threads showing the quart pots of hydroton in the grow log section, but I run vertical NFT. All it would take to swap over is rearrange the tubing and set the pump on a timer.