Hello and welcome to Apple Support Communities, keydet68!
I understand you want to know how to listen to music and view previously downloaded movies on macOS Catalina. I'd be happy to provide information to help you accomplish this.
You would simply have to set the media location on both the Music app and Apple TV app. To set this in the Music app, go to Music Preferences > Files and change the media location to your external drive. Similarly, in the Apple TV app go to Apple TV Preferences > Files and set it to your external drive.
Have a great day!