Why is my Apple Cash not applying to purchases and going on my card?

Apple Cash card/account should be in you Apple ID Account as a payment source. Drag the app to the top of other payment sources in your account. If you require specific steps, please post a reply and I’ll list the steps. This support article may help as well.

Add a payment method to your Apple ID - Apple Support

Re: "Why is my Apple Cash not applying to purchases and going on my card? When I make an in-app purchase,

my Apple Card is being used instead of my Apple Cash, which is listed first…. Why?"

Seems Apple Cash exists mainly to allow for exchanging money back and forth with people you know and trust well, or for transferring money in Apple Cash to your bank account or debit card.

Send and receive money with Apple Cash - Apple Support

or

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

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In-app purchases, and Apple purchases, would be made using a payment method listed in your Apple ID account.

Your Apple ID is the one account you use to access Apple services like the App Store, Apple Music, iCloud, iMessage, FaceTime, and more. It includes the email address and password you use to sign in as well as the contact, payment, and security details you use across Apple services.

You can check which payment methods Apple can accept in your country or region:

Payment methods that you can use with your Apple ID - Apple Support