How do you pay for an online purchase with the Apple Card?

See the following Apple Support article for information on finding your Apple Card number: How to find the card numbers associated with your Apple Card - Apple Support

Also see the following from How to make purchases with Apple Card - Apple Support

Pay with your virtual card number

If you need to give a card number when you check out online or in stores where Apple Pay isn't accepted, you can find your virtual card number in the Wallet app. If you use Apple Card Family2 and you share your account with a co-owner3, you each have your own virtual card number.

Your virtual card number is different from your titanium card number and Apple Pay card number.

titanium is the physical Apple Card. It is made of titanium rather than the plastic commonly used for credit cards. Apple Card includes both the virtual card (e.g., in the Wallet on your phone) and the physical card. Unlike other credit cards, the physical (titanium) card is optional. You only get one if you request it.