Apple doesn’t handle your funds. They are not a bank and that’s not how Apple Pay works. Apple Pay is an extremely secure method transmitting encrypted data and a one time use token. Apple Pay replaces the connection between the merchant and the bank when you initiate a transaction. Apple never transmits or handles you funds. Any approvals or declines are from the bank that issued the card used to fund the transaction.
When a refund is initiated between you and the merchant, the merchant uses the last 4 digits payment to identify the transaction. The last 4 digits are printed on the merchant receipt and are also in your Apple Wallet app.
If the merchant approves the refund, funds are transferred from the merchants account to your credit card account. Again, Apple has no part in the refund.
If you’re having an issue with your refund, either the merchant or the bank are delaying etc. Legally refunds may take 30 days, but that’s poor customer service and the merchant generally tries to avoid that.
If you’re curious about your refund, please contact both the merchant and your bank. Apple had no part in the refund process and can’t influence either participant. Good luck and I hope you receive your refund very soon!