It’s not rocket science. It’s the steamer wand in your espresso coffee maker.
Your espresso coffee maker is equipped with a steamer wand which passes steam generated in the boiler or thermoblock. This steam can be used to heat up any liquid simply by inserting the wand into the liquid and revving up the steamer. One major advantage of having an espresso maker with a steaming wand is that you can steam up the milk for that frothy texture externally. There is no milk running inside the machine and that saves a lot of cleaning after every use. Milk running in the machine is tricky; if not cleaned up properly, it can cause a build-up of bacteria.