How do they grow?

Early on I went on an organised walk in the cemetery- bird watching and listening to bird sounds. I think the highlights were watching a crow trying to drive away a pair of sparrow hawks. And watching a song thrush through my binoculars - it’s little beak wide open, singing loudly. He also taught us how to recognise chiff chaff, and black cap songs.
On the way there and back, in the warm sunshine I was noticing the various plants which grow without any soil, or water, out of cracks in walls or the pavement. It hasn’t rained for weeks. Valerian, fleabane, ivy leaved toadflax and campanula. How do they do this? In the afternoon it did rain a bit, but not enough to really soak the ground.