How to clear navigation Stack after navigating to another fragment in Android

I think your question specifically pertains on how to use the Pop Behavior / Pop To / app:popUpTo (in xml)

In documentation,
Pop up to a given destination before navigating. This pops all non-matching destinations from the back stack until this destination is found.

Example (Simple Job hunting app)
my start_screen_nav graph is like this:

startScreenFragment (start) -> loginFragment -> EmployerMainFragment

                            -> loginFragment -> JobSeekerMainFragment

if I want to navigate to EmployerMainFragment and pop all including startScreenFragment then the code will be:

        <action
            android:id="@+id/action_loginFragment_to_employerMainFragment"
            app:destination="@id/employerMainFragment"

            app:popUpTo="@+id/startScreenFragment"
            app:popUpToInclusive="true" />

if I want to navigate to EmployerMainFragment and pop all excluding startScreenFragment then the code will be:

        <action
            android:id="@+id/action_loginFragment_to_employerMainFragment"
            app:destination="@id/employerMainFragment"

            app:popUpTo="@+id/startScreenFragment"/>

if I want to navigate to EmployerMainFragment and pop loginFragment but not startScreenFragment then the code will be:

        <action
            android:id="@+id/action_loginFragment_to_employerMainFragment"
            app:destination="@id/employerMainFragment"

            app:popUpTo="@+id/loginFragment"
            app:popUpToInclusive="true"/>

OR

        <action
            android:id="@+id/action_loginFragment_to_employerMainFragment"
            app:destination="@id/employerMainFragment"

            app:popUpTo="@+id/startScreenFragment"/>

First, add attributes app:popUpTo='your_nav_graph_id' and app:popUpToInclusive="true" to the action tag.

<fragment
    android:id="@+id/signInFragment"
    android:name="com.glee.incog2.android.fragment.SignInFragment"
    android:label="fragment_sign_in"
    tools:layout="@layout/fragment_sign_in" >
    <action
        android:id="@+id/action_signInFragment_to_usersFragment"
        app:destination="@id/usersFragment"
        app:launchSingleTop="true"
        app:popUpTo="@+id/main_nav_graph"
        app:popUpToInclusive="true" />
</fragment>

Second, navigate to the destination, using above action as parameter.

findNavController(fragment).navigate(
     SignInFragmentDirections.actionSignInFragmentToUserNameFragment())

See the docs for more information.

NOTE: If you navigate using method navigate(@IdRes int resId), you won't get the desired result. Hence, I used method navigate(@NonNull NavDirections directions).