Check out this article to learn more about Chess960: What is Chess 960 or Fischer Random Chess?
Mobile
You can follow these steps on both the iOS and Android apps:
Tap the 'Play' button on your homescreen.
Tap the 'More' dropdown menu and select 'Custom:'
On the custom game screen, tap on 'Standard' and select 'Chess960' from the menu.
Tap 'Play!' to start your game!