App Store, Google Play and Windows Store are different platforms which don’t share information about your purchases – that’s why it’s impossible to transfer payment and activate PRO features. That means that if you have Piano Companion PRO in Google Store, you have to purchase it in App Store to get it in iOS.
At this moment PianoCompanion doesn’t support Univeral Purchases introduced on March 2020. That means that PRO for iOS and MacOS should be purchased separately.
However, you can share app within store.