If you still see "Downloading..." when you open the game, please leave your device and game open for up to 5 minutes and make sure you have at least 3 GB of free space available.

The game has to be open in order for the downloads to continue, as they can't be completed in the background.

This typically happens after an update, where everything has to be downloaded again, and is determined by your internet connection as well. So be sure you have the free space and a strong connection while doing this!