Choose the bundle identifier for an iOS and Mac app
Apple now allows you to use a single bundle ID for Mac and iOS apps through Universal Purchase:
Universal Purchase for Mac Apps Now Available
The macOS version of your app can now be included in a universal purchase, allowing customers to enjoy your app and in‑app purchases across iOS, iPadOS, macOS, watchOS, and tvOS by purchasing only once. Get started by using a single bundle ID for your apps in Xcode and setting up your app record for universal purchase in App Store Connect.
https://developer.apple.com/news/?id=03232020b
Offering Universal Purchase
Upload your apps to the app record using a single bundle ID. While the bundle ID must match the bundle ID you enter in App Store Connect, the apps can have different version numbers and build strings.
https://developer.apple.com/support/universal-purchase/
Rather a request for an opinion than a question I'd say, but com.mycompany.application.platform is what I'd use.
The reason would be to make it more and more granular towards the end, seeing the platform as a sub component of the actual application.