Let’s say you need to create a slideshow, then add some audio that plays with the slides, and output it as a Movie.

Here’s one way to do that.

Create the slides using Google Slides or Keynote

Record your screen.

Call up the slides. Start the presentation. Record your audio while advancing the slides at the right time.

Once done, press the record button on the top right [or call up Control Center and stop the recording there].

Then you can edit your creation in iMovie – don’t forget to make the audio louder!
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Record your screen

1. Go to Settings > Control Center > Customize Controls, then tap  next to Screen Recording. [That’s to make sure the button in step 2 is there]
2. Swipe down from the top right edge. 
3. Press and hold on  and tap Microphone. Make sure the microphone is set to ON.
4. Tap Start Recording, then wait for the three-second countdown. 
5. Open Control Center and tap . Or tap the red status bar at the top of your screen and tap Stop.

The file is stashed in your Photos.

Another explanation of the same thing [but more pictures]:
https://www.imore.com/how-record-your-iphone-or-ipads-screen-ios-11