What if there is no soil (concrete, gravel, clay)?

You can grow right in compost. Build boxes or raised beds on top of the gravel or concrete, lay a sheet of plastic in the bottom with a few centimetres of water kept there as a reserve, and fill with at least 30 cm of compost. Make it from kitchen scraps, leaves, grass, cardboard and seaweed if you have it. Leftovers of kvass, kefir or sourdough bring yeasts and lactic bacteria and speed things up; crushed charcoal gives microbes a home and holds nutrients; worms turn it into compost in about a month in the warm season. Top up with fresh compost and mulch every year, as plants eat it down.