how do I get the Apple Fitness + app on my TV? it's not showing up

Do you have an Apple TV set-top box of 4th generation or later with tvOS 14.3 software?

Devices you can use with Apple Fitness+ - Apple Support

Identify your Apple TV model - Apple Support

The Apple TV app isn’t used for this service. Unfortunately, smart TVs are not currently supported devices for this service. I’m not sure if it would be even possible (safely), as the Apple Watch would have to be paired with the device and communicate with the Fitness app securely. It is not just workout videos, it is the combination with your personal statistics.