When you purchase a downloadable item, you have unlimited access to your purchase and can download it as many times and as often as needed.
For videos, we recommend using VLC Player. It is a free download. Here is a link:
You will need to have the iOS version, 13.3.1 or newer, to be able to download directly onto the iPad. If you have an older version the videos will need to be directly downloaded from the iTunes iPad App to the iPad. To save your video download to your iPad, Apple suggests the procedure of downloading videos to a home computer and then transferring (Syncing) to the iPad
Here is a link to the Apple site for using iTunes to sync a video to an iPad: