Some content available on iTunes, but not TV App
I wanted to purchase Planet Earth III (aka Planet Earth Season 3), but it was not available in the TV App on any of my Apple devices. There was a box set that only included Season 1 (Planet Earth) and Season 2 (Planet Earth II), and those I already have, but individual seasons were not available. Fortunately, I have an older Mac on which I am still using iTunes, and I was able to find this there. Supposedly all of the content that is/was in iTunes will (someday) be available in the TV App, but until that time, there are bound to be other shows that are just not available at this time thru the TV App, but are still in iTunes somewhere. Hopefully Apple will get this fixed one of these days.
Apple TV 4K, tvOS 18