How do I transfer Apple Card funds to my external bank account

Assuming that you mean Gift Card funds, you generally can’t.

Gift Card funds can be thought of as “pre-spent” with Apple and can only be exchanged for Apple Goods & Services as described here:

What you can buy with your Apple Gift Card or Apple Account balance - Apple Support

If you REALLY mean that you have a credit balance on your AppleCard MasterCard; contact Goldman Sachs thru your Wallet app for assistance.

If you REALLY mean AppleCash … see this article:

Transfer money in Apple Cash to your bank account or debit card - Apple Support

An AppleCard Mastercard does not provide Cash Advances.

A credit balance may be refunded if you specifically request it from G.S. (otherwise it will normally be carried-forward and applied to the next billing cycle)