Below are some recommendations for learning the various technologies. These things are always changing.
If you want to build for Android phones/tablets you have to decide wether you want to build a native app (Java or Kotlin) or use one of the cross-platform libraries.
Google has the best videos and tutorials on Android. Go to developer.android.com and click on docs
in the top. If you want more structured "classes" they also have online classes as well as a set of online training courses.
If you want to build a mobile app that will run on both Android and iOS phones, then you must use a cross-platform framework. Some of these basically let you build a JavaScript app and then create a "native app" by bundling your javascript with a mini webbrowser just for that app. Flutter makes your Dart code run on either platform.