Hola Nicole,

Thanks, I hope you are OK too. 

In this lesson you mention we are mainly talking about a type of impersonal sentence using the they form without considering the subject as something relevant, e.g. "Llaman a la puerta." -> Someone is knocking on the door. This type of sentence is not a passive sentence. We can't put it into a passive and if we do it would sound very weird: "The door was knocked by someone ??"

These specific impersonal sentences are called "oraciones impersonales eventuales u ocasionales". 

I hope this helps,

Inma