How to transfer apple cash to apple card

Here you go:

How to make Apple Card payments - Apple Support

You can use your Apple Cash balance to make an immediate one-time payment. If the balance of your Apple Cash account is less than the amount of your payment, the remaining balance is paid from your default bank account.

You can also choose a different bank account to pay your Apple Card balance:

1. Open the Wallet app on your iPhone and tap Apple Card.
2. Tap Pay.
3. Choose a payment amount and tap the Pay [amount] button.
4. Tap your bank account, then choose a different bank account.
5. Tap the Back button and authenticate with Face ID, Touch ID, or your passcode to make a payment.