To open an app, tap its icon on the home screen, or swipe up from the bottom of your home screen to open the app tray, then tap the app.
To leave an app and go to your home screen, just swipe up from the bottom of the screen.
You don't need to close apps. Android manages the memory and battery that apps use, so you don't need to worry about leaving apps open.
To close one app, drag up from the bottom of the screen, hold, then let go. In your recent apps, swipe up on the app.
To close all apps, drag up from the bottom of the screen, hold, then let go. In your recent apps, swipe right until you see the start of the list, then tap Clear all. (You can prevent certain apps from being cleared.)
The task bar lets you to open other apps when you’re using an app.
Tap an app’s icon to open the app.
Touch & hold an app's icon for Split screen. It will open next to the currently opened app.

You can adjust settings to control how the task bar appears when you’re in an app:
To switch between recently used apps, swipe left or right at the bottom of the screen.
To see your list of recent apps, drag up from the bottom of the screen, hold, then let go:
You can clear all apps from your list of recent apps. Or, you can prevent an app from being cleared from your recent apps when you tap Clear all to close all apps.
To remove all apps from the list:
To lock an app in the list:

To unlock the app so it's removed when you tap Clear all:

Tap the search icon 
If you don’t find what you need, most apps include built-in help. To find it, open the app, tap 
