I subscribed for Apple TV through my smart tv. How can I add the subscription to my other smart tv’s and iPhone and iPad devices?

The iOS devices will have the TV App. Open the app on a device and sign into your account.

For the smart TVs you may need to download the Apple TV app to the television and then you can sign into your account.

See Apple's instructions at All the ways to watch Apple TV+ and more

The subscription is for an Apple ID account, and available on any of your devices, when signed in with that same Apple ID account.