Apple had to make compromises to give the iPhone X its enormous screen: namely, by removing the Home button. And since the Home button was used for so many different functions, anyone who gets the new device now needs to go back to interface school and learn the new methods.
We look briefly at all of the new gestures and remapped functions in our broad How to use an iPhone X roundup article, but here we drill down specifically into one aspect of this: how to change from one app to another on an iPhone X. Of course, you can could simply go back to the Home screen (swipe up from the bottom of the screen!), find the app icon and tap to open it; but it's much easier if you know how to bring up the app switcher, which used to be done by double-pressing the Home button.
How to bring up the app switcher screen
If you'd like to see a fair number of recently opened apps, then choose which one you'd like to jump to, the best approach is to use the app switcher.
Swipe up from the bottom of the screen (on the little bar, sometimes known as the 'Home indicator', that usually sits at the bottom of the screen, although it vanishes when watching fullscreen video and the like). It's the same gesture you'd use to go back to the Home screen, but this time keep your thumb on the screen for a moment - after a short while the app switcher screen will appear.
You can now swipe left or right to scroll through your recently opened apps, then tap one to open it.
Quick-jump to previous apps
Here's a quicker method. You don't get to browse multiple apps at once, but it's an easier way of flicking back to recent apps.
To jump back to the previous app, swipe across the bar at the bottom of the screen, from left to right. Some people are finding it most responsive if you do a little semi-circle gesture up, across and then down, but as with all gestures you should experiment to find what works for you.