I am unable to access the special features of the movies that I’ve purchased on Apple TV. How can I access them?

only available on apple tv boxes

iPhone, iPad, iPod touch, Apple TV box, Mac, and PC with iTunes.

I noticed that my special features for Tenet are not accessible

That title is said to include iTunes Extras (in my region anyway), if you purchased it from the iTunes Store. The special features would show by addition of a main menu, where you can select to play the main feature or the extras. You can contact the iTunes Store staff if they should show but do not.

iTunes Extras aren't available in the Apple TV app on smart TVs and third party streaming devices.

If you can't view your iTunes Extras - Apple Support