Apple uses security features built-in to the hardware and software of your Apple devices to help protect your transactions. When you make a purchase, Apple Pay uses a Device Account Number and a unique transaction code. Each transaction is authorised by your Touch ID, Face ID or device passcode. Your full card number is never stored on your device or on Apple servers, and it can’t be shared with merchants. Apple Pay doesn’t keep transaction information that can be tied back to you, ensuring all purchases are private.
More information on Apple Pay security and privacy:
https://support.apple.com/en-au/HT203027