Which built-in app is used to set up Apple Pay?

The Wallet app is the built-in application used to set up and manage Apple Pay on Apple devices. This app serves as a digital wallet where users can store not only their debit and credit cards for making payments but also boarding passes, tickets, loyalty cards, and more.

When users want to set up Apple Pay, they navigate to the Wallet app, where they can add their payment cards by scanning them or entering the information manually. The Wallet app provides a simple and secure way to access these payment methods and facilitates transactions in stores, apps, and online.

The other options do not exist as official Apple apps. "Apple Pay app," "Mobile Payments app," and "Money app" are not recognized applications within the Apple ecosystem, and therefore cannot be used for managing Apple Pay. This clarity reinforces that the Wallet app is the correct answer, as it is specifically designed for this purpose within Apple’s range of services.