To record, tap the red circle in the corner. Tap it again to stop and save to your Photos.

*NOTE: Long recordings will take several minutes to save and appear in your Photos.

Apple will ask for your permission to record the screen and microphone, be sure to grant permission.

In-app recording is a new feature. You can also use ‘screen recording’ to record instead:  How to use iOS Screen Record | (Video Guide)

Screen recordings have no sound? Enable Mic in Screen Recorder