Switching between apps on iPhone and iPad has gotten easier over the years with the HOME button. But with the iPhone X and iPad, the Home button is no longer there.
While in one app, Swipe along the bottom of the device to get into another open app. You can go through each app until you get to the one you wish.
Another way is, while inside an app, swipe UP from the bottom and stop in the middle of the screen. All of the current open apps will appear. Swipe left or right until you find the app you want. Tap on it to bring it to the foreground.