How do you add money to Apple Pay from apple balance

Apple Pay and Apple ID Balance (Apple Balance as you put it) are different systems.

Apple Pay is a payment system using your credit card and your iPhone or Apple Watch.

See Make purchases using Apple Pay

Apple ID balance is a credit system for purchases from Apple.

From Payment methods that you can use with your Apple ID

Your Apple balance is used - To buy apps from the App Store, use Apple subscriptions like iCloud+ and Apple Music, or buy other media from Apple

See the list of what you can purchase here.

You can check the value of your credit using these steps.

Apple notes in Check your Apple ID balance

You can't redeem or return Apple Gift Cards, App Store & iTunes Gift Cards, or unused Apple ID balance for cash, except as required by law.