Why need the variable currentScreen to be assigned two times in Android official compose sample project?

The Code A is from the Android official compose sample project.

1: I don't understand why author need to assign the varaiable currentScreen for the two times in Code A, and it seems that both Code B and Code C can work well, could you tell me why?

2: In Code A onScreenChange = { screen -> currentScreen = RallyScreen.valueOf(screen) }, where is the paramter screen (It's a sting) assigned?

3: Is Code A a good design? It's hard to understand. If Code C is correct, I think the Code C is easy to see, how about my thinking?

Code A

@Composable
fun RallyApp() {
    RallyTheme {
        val allScreens = RallyScreen.values().toList()
        var currentScreen by rememberSaveable { mutableStateOf(RallyScreen.Overview) }
        Scaffold(
            topBar = {
                RallyTabRow(
                    allScreens = allScreens,
                    onTabSelected = { screen -> currentScreen = screen },  //It's One time
                    currentScreen = currentScreen
                )
            }
        ) { innerPadding ->
            Box(Modifier.padding(innerPadding)) {
                currentScreen.content(
                    onScreenChange = { screen ->
                        currentScreen = RallyScreen.valueOf(screen)      //It's Two time
                    }
                )
            }
        }
    }
}


enum class RallyScreen(
    val icon: ImageVector,
    val body: @Composable ((String) -> Unit) -> Unit
) {
    Overview(
        icon = Icons.Filled.PieChart,
        body = { OverviewBody() }
    ),
    Accounts(
        icon = Icons.Filled.AttachMoney,
        body = { AccountsBody(UserData.accounts) }
    ),
    Bills(
        icon = Icons.Filled.MoneyOff,
        body = { BillsBody(UserData.bills) }
    );

    @Composable
    fun content(onScreenChange: (String) -> Unit) {
        body(onScreenChange)
    }
    
    ...  
}

Code B

@Composable
fun RallyApp() {
    RallyTheme {
        ...
        ) { innerPadding ->
            Box(Modifier.padding(innerPadding)) {
                currentScreen.content(
                    onScreenChange = {null }   // I changed 
                )
            }
        }
    }
}

...//The same as Code A

Code C

@Composable
fun RallyApp() {
    RallyTheme {
        ...
        ) { innerPadding ->
            Box(Modifier.padding(innerPadding)) {
                currentScreen.body()
            }
        }
    }
}


enum class RallyScreen(
    val icon: ImageVector,
    val body: @Composable () -> Unit  //I changed
) {
    Overview(
        icon = Icons.Filled.PieChart,
        body = { OverviewBody() }
    ),
    Accounts(
        icon = Icons.Filled.AttachMoney,
        body = { AccountsBody(UserData.accounts) }
    ),
    Bills(
        icon = Icons.Filled.MoneyOff,
        body = { BillsBody(UserData.bills) }
    );
   ...
}

from Why need the variable currentScreen to be assigned two times in Android official compose sample project?