Double taping side button

So, since now you know from my previous post that how emergency numbers are dialled, let us understand how to install apps using side buttons

Learn the basics

Use the side button to turn on your iPhone X, put it to sleep, use Siri, Apple Pay, and more.

Power on or off

To turn on, press and hold the side button until the Apple Logo appears.

To turn off, simultaneously press and hold the side button and either volume button until the slider appears, then slide to power off.

Wake and sleep

Raise to wake or tap to wake your iPhone X.

To put your iPhone X to sleep, press the side button.

Use Siri

Say "Hey Siri." Or press and hold the side button.

Install apps

After selecting an app in the App Store, double-click the side button to install it.

Use Apple Pay

To authenticate purchases with Apple Pay using Face ID, double-click the side button, then glance at your iPhone X.

Use gestures to navigate your iPhone X and later - Apple Support

https://support.apple.com/en-us/HT208204

The process of calling emergency is:

1. Press and hold the side button and one of the Volume buttons until the Emergency SOS slider appears. 
2. Drag the Emergency SOS slider to call emergency services. If you continue to hold down the side button and Volume button, instead of dragging the slider, a countdown begins and an alert sounds. If you hold down the buttons until the countdown ends, your iPhone automatically calls emergency services.