It was my understanding that Google / Apple do not charge a fee on in-app purchases which are not platform specific and which use third-party payment processing

It was my understanding that Google / Apple do not charge a fee on in-app purchases which are not platform specific and which use third-party payment processing. Never having played Fortnite, I assumed that in-app purchases are specific to your account and would carry-over if you moved from Android to iOS. Is that not the case?