I check the source code of BottomNavigationView.java.

In BottomNavigationView.applyWindowInsets(), engineers avoid the issue of overlapping with the system navigation bar by adding padding to the bottom.

https://github.com/material-components/material-components-android/blob/master/lib/java/com/google/android/material/bottomnavigation/BottomNavigationView.java#L142

The BottomNavigationView component is displayed above the system navigation bar, which seems to be in line with the MaterialUI design requirements.