ImmersiveMode.java

. . . 
  // other imports
+ import android.view.View;

+ import com.facebook.react.bridge.ReactMethod;

public class ImmersiveMode extends ReactContextBaseJavaModule {
  
    . . . 
      
  public ImmersiveMode(ReactApplicationContext reactContext) {
    super(reactContext);
  }

  // our annotation
+  @ReactMethod
  // methods annotated with ReactMethod must be public void
  // the name of this method is a bit of foreshadowing
+  public void fail() {
    // here we call our private method setSystemUIFlags
+    setSystemUIFlags(
      // the View class defines a whole mess of constants. 
      // If you examine the Javadoc for these constants, you'll see that each of these constants is an integer
      // the pipe symbol in Java is a bitwise OR operator.
      // By combining each of these flags using bitwise OR, we're effecitvely toggling
      // a single bit in the resultant integer. This will resolve to an integer that maps to a set of settings.
      // Neat, huh?
+      View.SYSTEM_UI_FLAG_IMMERSIVE
+      | View.SYSTEM_UI_FLAG_LAYOUT_STABLE
+      | View.SYSTEM_UI_FLAG_LAYOUT_HIDE_NAVIGATION
+      | View.SYSTEM_UI_FLAG_LAYOUT_FULLSCREEN
+      | View.SYSTEM_UI_FLAG_HIDE_NAVIGATION
+      | View.SYSTEM_UI_FLAG_FULLSCREEN
+    );
+  }
      
  private void setSystemUIFlags(int visibility) {
    View decorView = getCurrentActivity().getWindow().getDecorView();
    decorView.setSystemUiVisibility(visibility);
  }
      
}