Because there is no longer a separate app for most platforms, your method for "downloading" it depends on the device you use: Introducing Watch, a New Platform For Shows On Facebook
Download Facebook Watch: Your Guide to the Ultimate Video Platform download facebook watch app
To , you simply need to install the standard Facebook App from your device's official store. While a standalone Facebook Watch app was previously available for TVs, Meta has since discontinued those dedicated versions, integrating the full "Watch" experience directly into the primary Facebook application and web interface. How to Access Facebook Watch on Every Device Because there is no longer a separate app