It takes a unique combination of button presses to cause your iPhone to restart, and while it's not immediately obvious, once you've learned the steps, a force restart is the quickest way to restart your iPhone if it's acting up.
Force Restarting iPhone X, iPhone XS, iPhone XS Max, and iPhone XR
- Quickly press and release the Volume Up button.
- Quickly press and release the Volume Down button.
- Press and hold the Side button until the Apple logo appears, then release the Side button.
Using the force restart process prevents you from having to shut the iPhone down entirely, which takes several more steps.
If you do want to shut the iPhone down, you can do so by going to the General section of the Settings app, scrolling down to the bottom, and choosing the Shut Down option.
You can also hold down the Volume Up button and the Side button at the same time to bring up the Emergency SOS interface that also houses a "slide to power off" option.