How I can retrieve current fragment in NavHostFragment?

Navigation does not provide any mechanism for getting the implementation (i.e., the Fragment itself) of the current destination.

As per the Creating event callbacks to the activity, you should either communicate with your Fragment by

• Having the Fragment register a callback in its onAttach method, casting your Activity to an instance of an interface you provide
• Use a shared ViewModel that your Activity and Fragment use to communicate.

Reference to the displayed fragment (AndroidX):

java

public Fragment getForegroundFragment(){
    Fragment navHostFragment = getSupportFragmentManager().findFragmentById(R.id.nav_host_fragment);
    return navHostFragment == null ? null : navHostFragment.getChildFragmentManager().getFragments().get(0);
}

kotlin

val navHostFragment: Fragment? =
        supportFragmentManager.findFragmentById(R.id.nav_host_fragment)
navHostFragment?.childFragmentManager?.fragments?.get(0)

Here nav_host_fragment is an ID of the fragment tag in your activity_main.xml with android:name="androidx.navigation.fragment.NavHostFragment"