Variants are available only on the web and cannot be played through the Chess.com apps. However, if you'd like to enjoy a variant game on your mobile device, you can access it through your device's browser and play from there!
How to play a Variants game on a mobile browser
Go to Chess.com
Tap on the menu (
) icon at the top-left corner:
Tap 'Play' and select '4 Player & Variants:'
Choose the variant that you want to play, customize your game, and press 'Play!'
Check out this article for more about variants: How do I play chess variants.