The downloaded file will be saved to FileSystem.documentDirectory + 'small.mp4' so to access it later you will need to have a reference to this file path, which would be something like: file://Containers/Data/Application/55651607-FE3D-4AA1-8106-1AA7AC8BEBBD/Documents/small.mp4.
A few options:
Use AsyncStorage to save a reference to read back later.
Use Redux / Redux persist (also with AsyncStorage) to hold the reference
You will need to read the files from that created directory as well, so: