I bought your mobile app for iOS, how can I transfer it to my Android device?

You can not. And this is not up to us.
Google and Apple do not allow the exchange of Apps, there is nothing we can do. If you want to have the same App on an Apple device and a Google device at the same time, the only solution is to buy it twice.

This also applies to the reverse: if you purchased the Android App first, you cannot "transfer" it to an iOS device.