How does footage you take on a circle lens camera turn into square or rectagle when you watch it on your TV?

Good question!

The image transmitted by the lens is actually circular, but it passes through a rectangular opening just before the film plane. This creates a rectangular image on the film because part of what the lens is transmitting is blocked. You are never reminded of this since the entire "previewing" system is constructed to show you what you will get, not what the lens actually "sees".