How will I have access to movies I bought through AppleTV+ should I decide to cancel?

The Apple TV+ subscription is completely separate from movies bought from the iTunes Store.

• As Apple TV+ is a subscription service, you will have access to this subscription content as long as you subscribe, but not after that.
• Movies that you bought are yours to view at your convenience “indefinitely” (as long as the service exists, and as long as the purchased content is available there).

After the free trial, the subscription auto-renews each month (with a subscription fee) until you cancel.