How do I buy apps from Apple pay
How do I buy apps from Apple Pay?
Question marked as
You don't; you buy them from the App Store. Read this document to learn how to download apps and games on your iPhone or iPad
Download apps and games on your iPhone or iPad - Apple Support
As for paying for them, read this support article Payment methods that you can use with your Apple ID - Apple Support
and Add a payment method to your Apple ID - Apple Support