On the apps row of your home screen, go all the way left and click on Apps
Click Get more apps
Go to the magnifying glass icon to search
Search for Downloader. You can use the keyboard or press the Google Assistant button and say "downloader"
This is the one you need to install. Once it is installed, DO NOT open it yet
Go to your Android TV device's Settings and select Device Preferences
Go to Security & restrictions
Click on Unknown Sources
Select Downloader and toggle the switch to Allowed
Go back to the Apps section of your home screen and open Downloader