How to sync Wallet across MacBook and iPhone

There is no direct sync.

You can add the same cards to both, and they will show the expenses in both places though.

If you have not added the cards on the Mac go to System Settings ➜ Wallet & Apple Pay ➜ Add Cards it should then show you the cards you have added on other devices. You can click on the checkmark next to the ones you want to add and then click on Next to add them.