Share in-app purchases between iOS and Mac app

It depends on how the iOS and macOS have been set up. Recently App Store Connect has made it possible for iOS, iPadOS, macOS and tvOS apps to make use of the same application bundle identifiers and to use the same in-app purchase identifiers. In this case, purchasing the in-app item in the application on one platform can be restored to an app on a different platform (does not apply to consumable purchase type) You can learn more about this in the Apple Developer article "Offering Universal Purchase"
<https://developer.apple.com/support/universal-purchase/>

However, if you are trying to implement such support for an existing iOS and macOS app, each which uses their own application bundle identifier, then the solution is described in the Apple Developer article "Offering a Subscription Across Multiple Apps".
<https://developer.apple.com/documentation/storekit/in-app_purchase/subscriptions_and_offers/offering_a_subscription_across_multiple_apps>
The article describes a more complex solution for handling a subscription offering among multiple app, however the same solution works for apps on different platforms. The key is to implement a server to grant a user access to the in-app purchase. When the initial purchase is made, the appStoreReceipt is saved to the server under the user account. As the user uses the app on another platforms or other devices, the user logs into the server where the in-app purchase can be validated.