Kivy: referencing widgets

my.kv

BoxLayout:
    NavBar:
        sm: screen_manager
    ScreenManager:
        id: screen_manager

your.kv

#:import NoTransition kivy.uix.screenmanager.NoTransition

BoxLayout:
    orientation: 'vertical'
    
    NavBar:
        sm: screen_manager
        name: 'nav_bar'  # if you actually need this, name is usually a Screen thing
    
    ScreenManager:
        id: screen_manager
        transition: NoTransition()
        
        SplashScreen:
            name: 'splash_screen'  # for a single-use screen, you can also set the name in the kv rule for the screen so you won't need it here
        
        WeatherScreen:
            name: 'weather_screen'
        
        OperatorPanelScreen:
            name: 'operator_panel_screen'

You can still have separate kv files for other things, but at some point you need to put together the root widget. The root widget can be defined in a separate kv file - usually with the name of your app, as we will autoload this. For an app named ScreenManagerApp we will automatically look for a file named screenmanager.kv in the same folder. If you wanted to be more dynamic about which screens are added, just can them out of the root widget kv and add the screens in Python. The app kv file is loaded before App.build() is called.

def build(self):
    self.root.ids.screen_manager.add_widget(SplashScreen())