Can’t watch movies with Apple TV+ subscription

Choose the Apple TV+ sidebar after opening the app. All the content you see there is free with the Apple TV+ subscription.

If you are finding the same movies on Tubi, you are not viewing Apple TV+ content. The movies/shows in the Apple TV+ tab are not available on other streaming services. The Apple TV+ subscription gives you the ability to watch all of the Apple content for free, not the content included with the other streaming services.

If you use the Home tab in the Apple TV+ app, you see content from other sources and those are not be included in your Apple TV+ subscription. For example, there is a Paramount+ channel that would require a subscription to that service to view their content.

The Home tab gives you a broad overview of all content you may have with other streaming services, similar to the Hulu experience, and the Apple TV+ tab gives you only the content you can view with your Apple TV+ subscription.

only option is “how to watch” and “purchase”

It seems you may be using a Fire TV, Android TV, or Google TV device with the Apple TV app. On those devices, users could get to see the text on How to Watch for Store movies, and how to make any purchases for buy or rent movies, or third party in-app subscriptions on a different supported device (e.g. computer or iPhone or iPad). This is about making the transaction only. Once the purchase is done, you can then watch that content on your Fire TV, Android TV, or Google TV device.

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

“2. In the US, you can subscribe to Apple TV+ on Amazon Fire TV and Prime Video, but you can't purchase other content from the Apple TV app. On Amazon Fire TV (in other regions), Android TV, and Sony TV devices, you can't purchase content from the Apple TV app. You can buy, rent, or subscribe on another supported device and then watch in the Apple TV app on your Amazon Fire TV, Android TV, or Sony TV device.”

The movies and other content that is included with your Apple TV+ subscription is listed elsewhere in the app, not in the Store section. The Store is for buying or renting content, and optional other subscriptions, separate from this Apple TV+ subscription. One app with multiple non-free services.