How can I setup two Fragments Side by Side and Communicate between them?

This is my first time using fragments and I started off creating a new Project and choosing "Navigation Drawer Activity" in Android Studio. My first step was to add the second fragment in 'content_main.xml" (shown below) and update the references to the original nav_host in MainActivity.java to the left fragment.

<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:app="http://schemas.android.com/apk/res-auto"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
app:layout_behavior="@string/appbar_scrolling_view_behavior"
tools:showIn="@layout/app_bar_main">
<fragment
android:id="@+id/nav_host_fragment_left"
android:name="androidx.navigation.fragment.NavHostFragment"
android:layout_width="0dp"
android:layout_height="match_parent"
app:defaultNavHost="true"
app:layout_constraintEnd_toStartOf="@+id/guideline"
app:layout_constraintLeft_toLeftOf="parent"
app:layout_constraintStart_toStartOf="parent"
app:layout_constraintTop_toTopOf="parent"
app:navGraph="@navigation/mobile_navigation" />
<fragment
android:id="@+id/nav_host_fragment_right"
android:name="androidx.navigation.fragment.NavHostFragment"
android:layout_width="0dp"
android:layout_height="match_parent"
app:defaultNavHost="true"
app:layout_constraintEnd_toEndOf="parent"
app:layout_constraintStart_toStartOf="@+id/guideline"
app:layout_constraintTop_toTopOf="parent"
app:navGraph="@navigation/mobile_navigation" />
<androidx.constraintlayout.widget.Guideline
android:id="@+id/guideline"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:orientation="vertical"
app:layout_constraintGuide_percent="0.50121653" />
</androidx.constraintlayout.widget.ConstraintLayout>

Below is MainActivity.Java

package com.example.test;
import android.os.Bundle;
import com.google.android.material.floatingactionbutton.FloatingActionButton;
import com.google.android.material.snackbar.Snackbar;
import android.view.View;
import androidx.navigation.NavController;
import androidx.navigation.Navigation;
import androidx.navigation.ui.AppBarConfiguration;
import androidx.navigation.ui.NavigationUI;
import com.google.android.material.navigation.NavigationView;
import androidx.drawerlayout.widget.DrawerLayout;
import androidx.appcompat.app.AppCompatActivity;
import androidx.appcompat.widget.Toolbar;
import android.view.Menu;
public class MainActivity extends AppCompatActivity {
private AppBarConfiguration mAppBarConfiguration;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
Toolbar toolbar = findViewById(R.id.toolbar);
setSupportActionBar(toolbar);
FloatingActionButton fab = findViewById(R.id.fab);
fab.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View view) {
Snackbar.make(view, "Replace with your own action", Snackbar.LENGTH_LONG)
.setAction("Action", null).show();
}
});
DrawerLayout drawer = findViewById(R.id.drawer_layout);
NavigationView navigationView = findViewById(R.id.nav_view);
// Passing each menu ID as a set of Ids because each
// menu should be considered as top level destinations.
mAppBarConfiguration = new AppBarConfiguration.Builder(
R.id.nav_home, R.id.nav_gallery, R.id.nav_slideshow,
R.id.nav_tools, R.id.nav_share, R.id.nav_send)
.setDrawerLayout(drawer)
.build();
NavController navController = Navigation.findNavController(this, R.id.nav_host_fragment_left);
NavigationUI.setupActionBarWithNavController(this, navController, mAppBarConfiguration);
NavigationUI.setupWithNavController(navigationView, navController);
}
@Override
public boolean onCreateOptionsMenu(Menu menu) {
// Inflate the menu; this adds items to the action bar if it is present.
getMenuInflater().inflate(R.menu.main, menu);
return true;
}
@Override
public boolean onSupportNavigateUp() {
NavController navController = Navigation.findNavController(this, R.id.nav_host_fragment_left);
return NavigationUI.navigateUp(navController, mAppBarConfiguration)
|| super.onSupportNavigateUp();
}
}

When I run the app I see "This is home fragment" on both sides. I can use the menu to select another menu option, and the left side changes to display the new fragment, and the right is still showing home. My issue is when I start trying to communicate between the two fragments. I was looking at the example below:

https://codinginflow.com/tutorials/android/fragment-to-fragment-communication-with-interfaces

However in their MainActivity there is the below code

fragmentA = new FragmentA();
fragmentB = new FragmentB();
getSupportFragmentManager().beginTransaction()
.replace(R.id.container_a, fragmentA)
.replace(R.id.container_b, fragmentB)
.commit();

I dont see this in my current MainActivity, just some NavController items that I assume is somehow functioning in a similar fashion.

So I have 2 questions:

Q1 - How is the sample project replacing the left side fragment when I click on a menu item?

Q2 - How can I communicate with the right side fragment from the MainActivity? I am thinking I should be able to use findFragmentById in MainActivity OnCreate() but this doesn't work.

HomeFragment homeFrag = getSupportFragmentManager().findFragmentById(R.id.nav_host_frag_right);

I am guessing that I need to modify the way the fragments are created? Any help is much appreciated!

You can either use a ViewModel class to which fragments can easily communicate and it will act as an intermediator or you can create an interface an implement that in your activity.Fragments cannot interact directly.
https://developer.android.com/reference/androidx/lifecycle/ViewModel
https://developer.android.com/training/basics/fragments/communicating

you can call those interface methods by something like this in the second case :

   (getActivity)ac.methodA() ac is a reference variable of interface and methods() is 
one of the methods of interface.