I have an app on iOS, can I transfer to Android or vice-versa?

When purchases are made, they are made on that device platform. There is no mechanism allowing transfer of apps between different platforms.

When you purchase an app from either app store, you are making the purchase from Apple or Google, NOT directly from Hema Maps. Apple and Google make the policies which govern the use of their respective app stores.

However, it is the same for both. When you bought the app originally, you bought it using a specific app store account for a specific platform. If you change platforms, you will need to purchase the app in the specific platform's app store.