how do i get apple tv on our smart TV that is connected to Comcast Xfinity?

The Comcast Xfinity media box itself doesn’t support the Apple TV app.

Select (newer) models smart TV support the Apple TV app natively. Other TVs will need a supported external streaming device to access the Apple TV app.

List of Apple TV app-supported smart TVs and third party streaming devices - Apple

Select (newer) models smart TV support AirPlay. So that you may stream from an iPhone to the TV. Other TVs will need a supported external streaming device to bring AirPlay receiving capability.

List of AirPlay 2-enabled smart TVs and third party streaming devices - Apple

All the ways to watch in the Apple TV app - Apple Support

Watch on your TV

Apple TV box (4K, HD, and 3rd generation) ﹥

Streaming devices ﹥

Smart TVs ﹥

Gaming consoles ﹥