If you use Mac Catalyst to build a Mac version of an iPad app, you use the same bundle ID as the iPad app by default so you can distribute it as a universal purchase that allows customers to purchase the apps together from the App Store. Just add the macOS platform to an existing iPad app record in App Store Connect, then upload the builds separately.
To distribute the apps separately, set a different bundle ID for the Mac app by unchecking the Use iOS Bundle Identifier checkbox, then create a separate app record for the Mac app in App Store Connect. If you have In-App Purchases or Subscriptions, recreate them for the Mac app in App Store Connect. Go to Create an in-app purchase and Create an auto-renewable subscription in App Store Connect Help.
If you created the Mac app built with Mac Catalyst using an earlier Xcode release than 11.4, the Mac app and iPad app have different bundle IDs. The Mac app bundle ID has a maccatalyst prefix followed by the iPad app bundle ID by default. Use the Derive Mac Catalyst Product Bundle Identifier (DERIVE_MACCATALYST_PRODUCT_BUNDLE_IDENTIFIER) build setting to configure your project:
If you add an app extension, set the app extension target Derive Mac Catalyst Product Bundle Identifier build setting to YES because the Mac App Store requires that every embedded executable bundle have a bundle ID that is prefixed with the parent bundle ID.
If you want to use universal purchase (use the same bundle ID for the Mac and iPad app), change the app Derive Mac Catalyst Product Bundle Identifier build setting from YES to NO.