how can family watch apple tv on smart tv?

Select newer models smart TV support the Apple TV app. Verify that their model is supported.

Unsupported TVs may connect an external streaming device. That can be an Apple TV box, or select third party streaming devices.

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

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

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

Family Sharing allows a small group of users to share services, including Apple TV+. With Family Sharing, each user accesses these services on their own Apple ID account. Each group member must have their own Apple ID account. To define the group, the ‘organizer’ invites these other group members by their Apple ID account. The organizer must be an adult, and must use an iPhone, iPad, iPod Touch, or Mac to create the group. This organizer doesn’t have to be the one with the subscription, that can be any of the group members.

Family Sharing - Apple

What is Family Sharing? - Apple Support

Set up Family Sharing - Apple Support