Back stack
The stack of screens the user has visited; opening a screen pushes it, Back pops it.
Definition
Navigation libraries keep one entry per visited destination, each with its own lifecycle, ViewModels and saved state. Options like popUpTo, launchSingleTop and saveState shape the stack so Back does what users expect: login and onboarding disappear after use, and bottom navigation tabs keep their own history.
In practice
After login, navigate(Home) { popUpTo<Login> { inclusive = true } } leaves only Home, so Back closes the app.