To capture video you will need to:

-Tap the bottom-right corner of the screen to make the menu appear.
-Tap the video camera icon in the bottom-right corner to begin recording.

Once you tap the icon to stop recording, your device may give you a prompt to save your video.

On iOS, these videos will not contain any of the game UI or labels. It will only capture what is happening in the world and what the characters are doing.

On Android, it is slightly different in that it will capture the UI, and grant you the option to capture your face and voice. This will require you to sign into a Google account if you have not already done so.