To restart your iPhone without the power button, you can shut down via settings, ask Siri for help, or let your battery die. Then, recharge or turn back on.

Eventually, I discovered that I could double-press the power button to launch the camera on my Samsung phone. It also works on Google Pixel phones and other devices.

Quick Answer: To restart your iPhone without the power button, go to Settings > General > Shut Down, then slide to power off. You can also ask Siri to restart your iPhone without the button.