Perhaps I can offer a slightly different explanation that will help you to understand what is going on. Any circuit no matter how complex can be replaced by a voltage source and a series resister. Think of a 12 volt power supply; rectifiers, filters and everything in it works exactly like a 12 volt source through a series resister the equivalent of the power supply’s impedance.
When he talks about removing the voltage source, he is saying that it has a resistance of zero and is calculating the value of the equivalent series resister for the overall circuit.