For iOS, upload a .zip or .tar.gz file containing your compressed .app bundle.
Your .app bundle must represent a simulator build of your app. After running in iOS Simulator via Xcode, look in ~/Library/Developer/Xcode/DerivedData/<project-name>/Build/Products/Debug-iphonesimulator/.
Alternatively, you may run xcodebuild -sdk iphonesimulator (if you use .xcodeproj) or xcodebuild -sdk iphonesimulator -workspace Sample.xcworkspace/ -scheme <your-scheme> -configuration Debug (if you use .xcworkspace) in your project directory, then zip the .app bundle in build/Debug-iphonesimulator/.
For Android, upload the .apk containing your app.
After your app is built, either via Android Studio or by running the command ./gradlew assembleDebug in your project directory, look in <project-name>/<module-name>/build/outputs/apk/.
For more information, please see our docs.