The app stores, like GooglePlay, iTunes and Amazon each sell a different version of apps exclusive to the device platform. They are in competition with one another, and do not share their sales or profits. That means a copy purchased from GooglePlay cannot be downloaded again for free to a Apple device using iTunes. A new copy will need to be purchased separately for the Apple device. Same if the situation is reversed. This also is their policy for all IAPs (In App Purchases) made from each store.
There is one exception. Infinite Flight Live can be used on either platform with a paid copy of the app. This means if you buy Infinite Flight for your Android and Apple devices, then purchase a Live subscription on one, it can be logged into with a single purchase on the other device. However this can only be used or logged in to on one device or the other at any given time. Subscriptions cannot be used on two devices simultaneously.