iPhone - keep your device close to the terminal and confirm the transaction using Touch ID or Face ID. You will see the "End" sign on the device screen.
Apple Watch - hold your device close to the terminal and press the side button twice. You will feel a slight vibration when the transaction is confirmed.
iPad - from your iPad device, you can make purchases through the Apps.
In applications - Press the Apple Pay button or select Apple Pay as a payment method. Purchases through Apps are confirmed by Touch ID or Face ID on iPhone or iPad. For the Apple Watch, press the side button twice.