how do you pay your tmobile bill with apple pay

Apple Pay doesn't give you cash back. Only the Apple Card (credit card) does that. You would use it the same way you use any credit card to pay your TMobile bill. Generally, there's a place in your account when you log in on the TMobile website to enter the card you want to use for auto-pay. TMobile should be able to help you with that.

Do you have the TMobile app on your phone? If so, check, to see if it will let you set up Apple Pay as your preferred payment there. If that doesn't work, contact TMobile.