Now the graph. navigation/PicaNavHost.kt (in the code column).

• rememberNavBackStack(Screen.Menu) — a stack starting at the menu.
• NavDisplay(backStack, entryProvider { ... }) — draws the top of the stack.
• entry<Screen.Menu> { MenuScreen(...) } — maps the Menu key to the menu screen.
• onCartClick = { backStack.add(Screen.Cart) } — the menu's "cart" button pushes Cart onto the stack.
• viewModel() creates one CartViewModel, shared by the menu and the cart. No Koin yet — we introduce it in lesson 6; for now viewModel() is enough (the ViewModel has no dependencies).

You need a small MenuScreen change — so it can navigate to the cart and add pizzas to it. Add the params and buttons:

@Composable
fun MenuScreen(
    cartViewModel: CartViewModel,
    onCartClick: () -> Unit,
    items: List<MenuItem> = sampleMenu   // the real list arrives in lesson 5
) {
    Scaffold(
        topBar = {
            TopAppBar(
                title = { Text("Pica Menu") },
                actions = { IconButton(onClick = onCartClick) {
                    Icon(Icons.Default.ShoppingCart, contentDescription = "Cart")
                } }
            )
        }
    ) { padding -> /* … LazyColumn of cards … */
        // in a card:
        Button(onClick = { cartViewModel.onEvent(CartEvent.AddItem(item)) }) { Text("Add to cart") }
    }
}

Finally — MainActivity from lesson 1: replace Greeting("Pica") with PicaNavHost(). Navigation is finally on.

Verify. Run it. You should:

1. MENU opens with pizzas + a cart icon in the top bar.
2. Tap "Add to cart" on Margherita → the cart shows it has something.
3. Tap the cart icon → CART opens with Margherita and total €8.50.
4. Press the phone's Back → back to the MENU.

Two screens, one flow.

Gotcha (Nav3 alpha). Because Navigation 3 is still alpha, expect small API mismatches: NavDisplay, entryProvider, rememberNavBackStack may live in a different package or under a different name in your version. If an import is red — don't guess, open the official Nav3 docs and check the exact name for your library version. This is normal with alpha libraries; the course code targets navigation3 1.1.4.