You can buy the passes at every Ventra Vending Machine, including those at O'Hare. The pass is valid for all CTA buses and trains, that includes the L.

More info here.

The Ventra app is only good for checking your balance, adding to the fare value and buying passes. You must have a Ventra card registered on the app to do that. You can't pay with the app. If you want to use your phone to pay the fare, you need to load the Ventra card onto Apple Wallet or Google Pay.