If you will look for an option to download Facebook videos on your mobile you will not get any option in the Facebook app because Facebook didn't have an option right now but i will tell you an alternative for this work:-

- First, open the video on Facebook and copy the link to that video

- Then go to any browser and visit fbdown.net

- After that paste your copied link and select your desirable quality

- Then you can click on the download button to start download

Also read: https://sites.google.com/view/getassistnet/social-media/facebook/Download-videos-from-Facebook