@State、@Binding、@ObservedObject、@StateObject、@EnvironmentObjectを比較してください。

// Value state OWNED by this view
struct Toggle: View {
    @State private var isOn = false           // local source of truth
    var body: some View {
        Switch(isOn: $isOn)                    // $ makes a Binding
    }
}

// A two-way reference to state owned ELSEWHERE
struct Switch: View {
    @Binding var isOn: Bool                    // no ownership; reads+writes parent
    var body: some View { Button("\(isOn)") { isOn.toggle() } }
}

final class CartModel: ObservableObject {
    @Published var items: [Item] = []          // change => views re-render
}

struct CartScreen: View {
    @StateObject private var model = CartModel()   // OWNS: created once, survives re-renders
    var body: some View { CartList(model: model) }
}

struct CartList: View {
    @ObservedObject var model: CartModel        // observes; owned by a parent
    var body: some View { Text("\(model.items.count)") }
}