Hello LeAnnB1218,
Apple Pay, Apple Cash, and Apple Card are all different functions.
The Apple Card works as both a physical and digital credit card. It is associated with your bank checking account in order to make payments. You cannot buy an Apple Card, only apply for one.
Apple Cash is associated with Apple Pay, but it allows you to send and receive funds through Messages from other people. Apple Pay allows you to use that Apple Cash balance or other cards stored in Wallet in physical and virtual stores that accept it as a payment method. As described in the link prior, "Using Apple Pay can be simpler than using a physical card, and safer too. With your cards stored in the Wallet app, you can use Apple Pay to make secure payments in stores, for transit, in apps, and on websites that support Apple Pay. In Messages, use Apple Cash to send and receive money from friends and family and to make purchases from participating businesses."
If you are looking to add funds to Apple Cash, then you'll want to visit: Add money to Apple Cash - Apple Support and that will explain how you can do so.
Thanks for using Apple Support Communities!