If you own the source code, you don't "convert" the file; you it for the other platform.
iOS relies on Apple-specific frameworks (like UIKit). Android uses Google frameworks. ipa to apk converter online
An IPA file (iOS App Store Package) is built for iOS. It contains compiled code written in or Objective-C , and it relies on Cocoa Touch — Apple's exclusive software framework. If you own the source code, you don't