Its very simple.
The blower motor on top of the engine blows air across the exhaust manifolds and into the cabin.
The 'heat exchanger' is an exhaust manifold with tinware enclosing it.
In the pic below air is blown into the hole facing you, over the manifiold piping, and out through the chimney looking bit at the back. From there it goes through a flap and into the car.
Hope this helps.
