what happens to movies you have bought if you do not renew your Apple TV subscription

The subscription content is different content from the buy/rent content. It is completely separate.

• The rentals are available during the rental period.
• The buy items are available as long as they are available in the iTunes Store.
• The subscription content is available as long as you subscribe.

The Apple TV+ subscription content cannot be bought or rented.

The buy/rent items are not part of the subscription.

If you purchase movies that has nothing to do with your Apple TV+ subscription

Apple TV subscription is a streaming service that allows you to watch all in the catalog for free.

Here is how to re download past purchases:

Redownload apps, music, movies, TV shows, and books - Apple Support

@ ammimac

Purchased content is yours to keep for as long as they’re still available in the iTunes Store (signed in with the same Apple ID you used for the purchases). It’s best to download them to a device or computer if you don’t want to lose them. I download most of my movies.

Sorry, I meant to reply to the OP