How do you properly build a genuinely adaptive navigation structure that automatically switches between a bottom navigation bar on a phone and a genuinely wider navigation rail or a permanent side drawer on a considerably larger tablet or desktop screen?

A genuinely adaptive navigation structure typically checks the current available screen width and conditionally renders a considerably more compact BottomNavigationBar on a genuinely narrower phone screen, while switching over to Flutter's own dedicated NavigationRail widget, which displays those exact same navigation destinations as a considerably narrower persistent vertical side rail, on a considerably wider tablet or desktop screen, taking genuine full advantage of that additional available horizontal screen space rather than wastefully consuming valuable limited vertical space with a bottom bar that would otherwise genuinely feel considerably less appropriate on a much wider display.

constraints.maxWidth > 600
  ? NavigationRail(destinations: destinations, selectedIndex: currentIndex)
  : BottomNavigationBar(items: bottomNavItems, currentIndex: currentIndex);