How do I transfer money to my Apple Card?

Apple Pay is not a credit or debit card. Apple Pay is a digital way to make payments using your credit and debit cards that you have placed into the wallet app.

The Apple Card is a Mastercard credit card. As such you cannot transfer money to it. Other credit cards operate in the same way.

The Apple Cash card is a digital prepaid debit card that you need to connect to a bank account in order to add money just like any other bank debit card.

As for setting a default payment card follow the instructions in the link provided below.

Set up Apple Pay in Wallet on iPhone - Apple Support

Set the default card for payments

The first debit or credit card you add to Wallet becomes the default card. To choose a different card, do the following:

1. Go to the Wallet app  on your iPhone.
2. Touch and hold the card you want to set as the default, then drag it to the front of the stack.

You find instructions on how do just about anything by searching Apple Support’s document database. They using it next time you have a question.

Official Apple Support