기본적인?(보편적인) activity.xml 레이아웃 구조

activity.xml

<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout 
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    ... >

    <com.google.android.material.bottomnavigation.BottomNavigationView
        android:id="@+id/bottomnav"
        ...
        app:menu="@menu/bottom_nav_menu" />

    <androidx.fragment.app.FragmentContainerView
        android:id="@+id/container"
        android:name="androidx.navigation.fragment.NavHostFragment"
        app:defaultNavHost="true"
        app:navGraph="@navigation/mobile_navigation" 
        ...
        />

</androidx.constraintlayout.widget.ConstraintLayout>