This device has a non-removable battery. This process can be used to simulate a 'battery pull' when the device is unresponsive, frozen or doesn't power on.
If your iPhone is powered on and responsive, refer to Restart/soft reset instructions.
If you are not able to restart your iPhone using these steps, check out this Apple Support page.
iPhones running iOS 13 or higher, may experience a delay before being able to power cycle the device back on after performing an alternative reset.
Press and hold the Power button.
Continue to hold the Power button and press and hold the Volume down button located on the left side of your iPhone.
Hold both buttons as the iPhone powers off and back on; release both buttons when the Apple logo appears on the screen.