This Youtube account (PeriscopeFilm) is a goldmine. I was recently recommended this film by the mighty algorithm:

"BASIC PRINCIPLES OF FREQUENCY MODULATION 1944 U.S. WAR DEPARTMENT FILM FM RADIO 86794"

https://www.youtube.com/watch?v=AzvxefRDT84

It explains how AM and FM radio works in great detail (30 mins).

Video uploader: https://periscopefilm.com/about-us/