By looking at the Material Design document about Bottom Navigation I can see multiple pages with nice transitions that use only one BottomNavigationBar, and probably one Scaffold.

I would like to replicate this but I struggle to do so, and previous answers lead to wrong solutions with bad animation and multiple Scaffold.

I want, one MaterialApp with multiple routes and one scaffold with one bottom bar. By tapping the bar items, I want to change the routes and so, the page and keep the actual bottom bar.

What is good the practice to implement this?