Poured the concrete slab today. Notice in this first picture that the tubing is arranged in smaller loops with a space between them. that's for different zones, and to allow the concrete expansion joints to go in without having a tube cross it. An expansion joint is a pre-planned crack, that allows the slab to expand and contract. Having a small joint there keeps the rest of the slab crack-free.
Notice the orange line in the picture above- that's the target concrete level. After the concrete is poured in, it's screed. This is done by drawing a straight edge along the top of the concrete to cut off the humps and fill in the hollows. it gives you a roughly level surface. In the picture below, they're using the painted line on the sidewall to guide their scree.