Back when I had an Ipad I purchased a lot of apps I'd already bought on Android and then after, I found new apps I liked on IOS and repurchased them for my Android devices. I don't think anyone likes this but that's the way it is and it's not likely to change. The systems don't communicate with one another and the developers would have to have a fairly extensive bit of automation to allow them to handle that themselves and I doubt that many people have devices in both platforms so it probably wouldn't be worth doing.
I have a better idea! Apple sells IOS to Google and the next Idevices come with Android installed.
Just kidding! I'm glad there's this competition. Each system would be worse without the pressure of the other one.
Barry