You have two controls, 1 for temp and one for pressure (instead of the traditional hot and cold with independent pressure controls on each). The temp knob has a stop in one position as you turn it, which gives 38°C water. The cheaper models are external and use a wax cylinder inside:

Wax expands as it gets warm. So when you first add pressure, internally the hot water valve is 100% open. Then as the water gets hotter at the valve, it closes and starts letting cold water in to retain the temp you select. It reacts quite fast too, so even if you have screaming hot water arrive at the fixture it’s quick enough to protect you from scalding. You set it and forget it. If someone else in the house flushes a toilet or runs a washer or dishwasher, thus causing the cold water to lose pressure, the thermostatic fixture will automatically adjust to maintain the temp.

The pricier ones are internal, which means all the complex valves are buried in the wall. And instead of wax they use a bi-metallic mechanism (which exploits the fact that a couple diferent metals bend differently with temp, or something like that). They look like this:

I think that has 2 pressure knobs, one for rainfall and one for the wand.

Whole external kits with rainfall and wand start at €100. If you just want the mixer bar and wand, it can be as cheap as €25. The internal kits start at around €500. I like the external ones not just because they are cheaper, but the plumbing is simple. You just need to have hot and cold threaded ½" pipes sticking out of the wall 15cm apart, which the fixture can screw onto.

It’s a valve that controls both the pressure and temperature of the shower water. It maintains a constant temp through your shower.

But installing one is 2x the price of a normal pressure-balance valve.