How can I find downloaded movies on my iPad?

You asked where the downloads are kept. The downloads are kept in each app.

There is no universal downloads folder if that is what you are asking. Each app holds its own downloads.

Anything downloaded within each app will be found in the app's specific downloads section. That's where they are, and that's where they can be found once they are completely downloaded.

You will not find them anywhere outside of the specific app you downloaded them to.

What streaming service did you download these from?

Each streaming app, will have its own Downloads location. Where it can be found in the specific App will depend on the streaming service.

For instance on Netflix the Downloads section is can be found by tapping on the downward arrow the top right corner. If you have no available internet connection when you open Netflix, it will open directly to the Downloads section.

On Prime video, the Downloads section can be found in the bar at the bottom of the screen. Again, when there is no internet connection it will open directly to that section.

You can try it out, by turning off Wi-Fi (and cellular if your iPad has it) and opening the app.