The blower heat exchanger heats water by ducting the high temperature blower exhaust over a series of copper tubes that the water flows through. As the exhaust heats the copper tubes, that heat is transferred to the water flowing through them. This is a common and efficient way to heat water.
This part is the core only of the Boxxer 421 HE system, it's the box of copper tubes with the hose fittings attached and the exhaust fins all contained in a sheet metal box that's riveted together.
Occasionally debris can build up on the fins of the HE core, this can cause unwanted back pressure on the engine and low temperature.