Mathieu Grenier

Firebase is a all in one package for mutiples API ready for you. Perfect for a small project and free. No need to code for a backend! Firebase give you almost evwrything to begin a new dev project! MongoDB is only a NoSql database, you must code backend separetely and others features. Firebase use also a NoSql database with Realtime or Firestore.

Kotlin is mostly for Android app but you can make crossplatform with Kotlin. Its more complicated...

Flutter is really easy to do a crossplatform.

Go with Flutter for a small/mvp projects. I have no knowledge with Dart. Its very easy to learn. Once learned, you are almost learned TS / JS . Its very similar.

Kotlin for big/advanced projects. Learning curve is more longer than Flutter.

You can start by small steps with Flutter and after Unity. Flutter = best choice to build a small cross-platform mobile app. With or no flutter, use directly Unity. Y'll have complete control but it's harder for new mobile developers. Keep in mind, the requirement is Unity!