How do I transfer money from my Apple Pay account over to my Apple Cash account?

There is no such thing as an “ApplePay account.”

ApplePay is the system which enables you to — among other things — use AppleCash, credit, and debit cards from your mobile device

There is possibly a Balance on your AppleID.

(we know it’s confusing at first)

However, any balance on an AppleID is generally non-refundable, and can ONLY be used to purchase Apple goods and services.

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