In this example we are going to see how to connect bottom navigation bar with android navigation which we have learned in Android-Navigation example. Do check it out first before moving here.

We have to create a bottom navigation bar and connect it to navigation or navigation graph so that user can easily navigate through different fragments of app by clicking options of bottom navigation bar.
Also we need to add top tool bar such that if user has navigated to different fragment other than homeFragment then they see a back arrow icon in top tool bar clicking on which they can come back to home fragment.

• Create different fragments in different packages inside application's package.
• For more info about creating fragments checkout fragment creation

• Setup navigation and its components navHostFragment, navGraph, navController as per the Android-Navigation
  Note : No <fragment /> of navigation resource file is interconnected. Check my_navs.xml

• To create tool bar add <androidx.appcompat.widget.Toolbar /> element to activity_main.xml.
• Set attributes of ToolBar properly, specially id (tool_bar for this example) and theme.
• In java file use Toolbar tb = findViewById(R.id.tool_bar); to get app's tool bar.
• Call setSupportActionBar(tb); which tells system to use tool bar as app's native action bar.

• To setup bottom navigation bar add <com.google.android.material.bottomnavigation.BottomNavigationView /> in activity_main.xml.
• Create a new resource folder in res/ named menu.
• Create a menu resource file in res/menu/
• Add <item></item> elements to the menu resource file with proper id and icon and title. for this example I have added home, profile, about items.
  Note that ID of menu item should be same as its corresponding destination ( element of navigation resource file) in navigation graph.
• Use app:menu attribute of bottom navigation bar to set menu resource file as options of bottom navigation bar.

• Obtain navController using NavController ctrl = Navigation.findNavController(findViewById(R.id.nav_host_frag));
• Obtain bottom navigation bar widget using BottomNavigationView bottom_nav = findViewById(R.id.bottm_nav_bar);
• Use setupWithNavController() of NavigationUi class to link bottom navigation bar with navigation controller as follows NavigationUI.setupWithNavController(bottom_nav, ctrl);
• Now as user will click different options of bottom navigation bar they will redirect to different pages/fragments of app.

• Obtain ToolBar using Toolbar tb = findViewById(R.id.tool_bar);
• Create AppBarConfiguration which have navController's navGraph linked with it using AppBarConfiguration app_bar_conf = new AppBarConfiguration.Builder(ctrl.getGraph()).build();.
• Use overloaded method setupWithNavController() of NavigationUi class to link ToolBar with navigation controller and AppBarConfiguration.
• Now as user will switch on to different fragment a back arrow icon will appear in top tool bar clicking on which they can come back to home destination.

This example is created with help of android docs, read them for more information. Some most used portion of android docs in creating this example given below :-

Setting up app bar - ToolBar

Tool Bar Android Widget

Connect navigation to ToolBar that allows user to go back

Connect UI components to bottom navigation bar