How to restart my iPhone 8plus

See the instructions below from Restart your iPhone - Apple Support

How to restart your iPhone SE (2nd generation), 8, 7, or 6

1. Press and hold the side button until the power off slider appears.
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3. 
4. Drag the slider, then wait 30 seconds for your device to turn off.
5. To turn your device back on, press and hold the side button until you see the Apple logo.