With other iPads, you can double-press the home button for the App Switcher, which displays thumbnails of all open apps.

On the iPad Pro, you have to swipe up from the bottom of the screen and hold your finger for a split second until the App Switcher appears. From there, swipe to the right to see your previous apps. You can also reverse direction by swiping to the left. When you spot the app you want, tap to open it. With iOS 12 or higher, swiping up and across to view your open apps works on all iPads.