What do you use to develop apps?

Actually depends on the specific needs and circumstances of the project. React Native developers might be more common given that it's based on JavaScript which is a very (the most?) popular language. However, Flutter is rapidly growing in popularity...

Mobile App Development: iOS (Apple): Xcode with Swift or Objective-C. Android (Google): Android Studio with Java or Kotlin.

I'm using Angular at the moment, but I'm shifting heavily into React.