For Touch ID, the intent to pay is confirmed using the gesture of activating the Touch ID sensor combined with successfully matching the user’s fingerprint. Details at Uses for Touch ID and Face ID - Apple Support
When you hold your iPhone near a POS which support contactless payment, the Wallet will appear on your screen right away. If you put your finger on the Home button (also the Touch ID sensor) at the same time, you had made a Touch ID process. Then your card on the screen will work with the POS and complete the transaction.