apple tv

If your smart TV has the Apple TV app, then you can watch Apple TV+ on that TV without an Apple TV box.

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

Get the Apple TV app or Apple TV+ app on your smart TV or streaming device - Apple Support

How to watch Apple TV+ and more - Apple Support

The Apple TV+ subscription content is available on any of your devices that has the Apple TV app, when signed in with the Apple ID account that has this subscription. The only limit is the number of simultaneous streams per subscription.

Compare Apple TV app vs. Apple TV box - Apple Community.

1. No such thing as an Apple TV+ device. The black box device sold by Apple is just an Apple TV (no plus).
2. If you mean you have logged in to an Apple ID that holds an Apple TV+ subscription on an Apple TV, that does not stop you from logging into the Apple TV app or Apple TV+ app on a smart TV with the same Apple ID and access the Apple TV+ subscription there.
3. You would only need another physical device, if your TV does not support the Apple TV or Apple TV+ apps.