If you are used to buying TV programs & movies in iTunes on Mac, you will notice that the program is missing with the upgrade to macOS Catalina. [1
If you subscribe to Apple TV +, the streaming service is in the TV app, where you can watch Apple's original shows, including Dickinson The Elephant Queen For All Mankind  Ghostwriter Helpsters Oprah's Book Club The Morning Show, See and Snoopy in Space .
If you now only update to Catalina, you will see a TV app icon in the dock. Here you can read how to use the Apple TV app.