In order to authorize an Apple Pay transaction, you must double click the side button, and the charge will be billed to the default card in your Wallet used for Apple Pay. You can view the default card used for Apple Pay at Settings > Wallet & Apple Pay > Default Card. You can view transaction details on that card by going to the Wallet app on your phone, where it will show the merchant name and amount.
Apple does not know what items you purchase with Apple Pay, that is something only the merchant that you authorized to pay with Apple Pay would know.
After reviewing the default card used for Apple Pay, go to your Wallet app and view the name of the merchant on that card. If you do not know what you bought contact the merchant. A charge will never happen by simply tapping a button on the screen. Along with choosing the Apple Pay option on the screen for your payment method of a purchase, it would require you to double click the side button to authorize the charge.