ImmersiveMode.java

public class ImmersiveMode extends ReactContextBaseJavaModule {
  
   . . . 
     // Previously added stuff

     // our method takes one argument, systemUIFlags. 
     // It ends up being an integer, which is pretty neat-- more on that later.
    private void setSystemUIFlags(int visibility) {
        // here we get ourCurrentActivity
        // from it, we grab the window
        // and from that we grab the decorView for our current activity
        View decorView = getCurrentActivity().getWindow().getDecorView();
        // think of the decorView as the eldest ancestor of the UI class familly tree.
        // it contains not only your views, but all current system views
        // eg system alerts and *navigation elements*
        // it contains a method for manipulating some of those system views
        decorView.setSystemUiVisibility(visibility);
    }
}