iTunes vs Apple one.

Yes, you still have to buy the content, e.g. movies in the iTunes Store, even if you have Apple One.

Apple One includes the Apple TV+ streaming service. With that app, you can stream the content that Apple TV+ contains, but NOT the content in the iTunes store.

Apple TV+ streaming service with its own content, just like Netflix.

iTunes is a store where you can buy and rent movies.

Apple One Individual and Apple One Family contain the same services, the difference is just that the "Family" version allows you to share those services with your family group.

It is not possible to share individual Apple One subscriptions. But if you have bought movies or music in iTunes, you will be able to share it with the members of your Family Sharing.

You can read what you can share and what you can't share here:

What types of content can I share with my family? - Apple Support