How can I withdraw money from my Apple Cash account and transfer it to Apple Pay?

Apple Pay is not an account that you can add money to, so the confusion lies in what you are trying to accomplish. What card are you trying to add money to from your Apple Cash card or are you trying to add it to your bank account?

If you literally mean Apple Pay (and not Apple Card), in other words, if you would like your Apple Cash to be Apple Pay’s default payment method, simply open Wallet and swipe the Apple Cash card to the bottom of the payment methods.