I found this TikTok but I didn’t quite grasp it. https://vm.tiktok.com/ZMYFDecSL/
Water does not instantly boil at 100 C, as in you can have liquid water at 100 C. To actually make the water turn to gas you have to keep pumping more energy into it.
This is what the burner at the bottom is doing, providing more energy to turn the water to gas. This is why the water at the bottom is what turns to gas and then bubbles up, causing what we call boiling.
The water that is up in the second container is not exposed to that energy getting added, because it’s away from the burner, it is only exposed to the water around it. That water around it is only at 100 C, meaning the water in the container can only get to 100 C, but cannot boil because there’s no more energy to add to it. It’s away from the burner.
It’s basically a “heating brake”:
If the big pot gets above 100, water evaporates, taking heat away, restoring the 100. The result is that the suspended container can’t ever get above 100.
The whole purpose is to guarantee you don’t burn what is in the suspended container. Very good to melt chocolate without paying too much attention. And you can get away with not stirring it.
At normal room pressure water boils at 100°C. The steam that is generated is also at 100°C. It can’t get hotter than that unless you seal it and allow the pressure to increase (which is how pressure cookers work).
In a heated vessel where you are heating the base, the top surface is cooling, and the liquid between the top and the bottom is going to sit somewhere between the cooler top surface temperature and the hot base temperature.
In a double boiler (or bain marie) situation the bottom of the inner container is only ever getting heated by the steam (or water) from he boiling lower container, so it only ever gets to 100°C, so it’s impossible for the liquid inside the upper container to get hotter than that.