Have you ever used a streaming service like Netflix, Hulu, HBO Max, etc.? This is very similar, except you are not paying a subscription fee -- you are paying a one-time fee to "own" a digital version of a movie.
There's nothing to download. Once you purchase the digital movie, you stream it through the Vudu app, which is available on most 3D TVs with "smart" capabilities, Apple TV boxes, several Blu-ray players, iPhones, iPads, Android devices, etc. On Vudu, 3D movies are in a side-by-side format, so you might need to manually turn on your TV/projector's 3D mode once the movie starts playing. You can stream 2D movies on the Vudu website, but you can't watch 3D movies from the website (you will get an error that says, "Cannot play this movie on this device").
On the Vudu app for iPhone and iPad (and I assume for Android phones & tablets), you can download the movie to the app for offline viewing. But you can only watch the movie through the Vudu app - you don't get access to the file and you can't transfer it to your computer. I don't think this feature is very useful for Vudu's 3D movies, since you won't be able to watch them in 3D on any phone or tablet that I know of.
Hmm. Sounds like I would "buy" the movie, but could never actually take sole possession of it. To me, that is not owning anything; it is merely the ability to watch repeatedly without incurring additional charges, for as long as the vendor decides to keep their site going, which could change at any time, and I would no longer have what I "own". Perhaps I am being too harsh in my analysis? Apologies if my innate cynicism is coloring my thoughts.