when a video is downloaded it is stored in the user's device. iOS is more restrictive regarding file system access, but in Android the users can access any of the downloaded files so you can't prevent them to share them.
The only solution I can think of would be to encrypt the videos when they're stored, and decrypt them when they need to be played. However, that solution would be quite complex and it would slow down the download and play of the videos. Also, since our code is open source, any advanced user could easily decrypt the video themselves.