“默认”底部导航视图是如何改变片段的？

bottomNavigationView.setOnNavigationItemSelectedListener(new BottomNavigationView.OnNavigationItemSelectedListener() {
    @Override
    public boolean onNavigationItemSelected(@NonNull MenuItem item) {
        FragmentManager fragmentManager = getSupportFragmentManager();
        FragmentTransaction fragmentTransaction = fragmentManager.beginTransaction();

        switch (item.getItemId()) {
            case R.id.navigation_home:
                fragmentTransaction.replace(R.id.fragment_container, new HomeFragment());
                break;
            case R.id.navigation_dashboard:
                fragmentTransaction.replace(R.id.fragment_container, new DashboardFragment());
                break;
            case R.id.navigation_notifications:
                fragmentTransaction.replace(R.id.fragment_container, new NotificationsFragment());
                break;
        }

        fragmentTransaction.commit();
        return true;
    }
});