It is important to note that apps built with frameworks like Flutter, React Native, or Xamarin can run on both Android and iOS. However, this is not done via a converter tool. The developer must possess the original source code and compile separate APK and IPA files directly from that source. If you are a developer with source code, you don't need a converter; you need an IDE (like Xcode or Android Studio).
Most major apps exist on both platforms: Spotify, WhatsApp, Netflix, etc. For niche apps, search for "iOS alternative to [App Name]." apk to ipa convert