複数モーダル（sheetやフルスクリーンなど）を開いている場合、一気に一番初めの階層に戻りたい時

以下二つのどっちか実行するだけで戻れる。

UIApplication.shared.windows.first?.rootViewController?.dismiss(animated: true)

または

let scenes = UIApplication.shared.connectedScenes
let windowScene = scenes.first as? UIWindowScene
let window = windowScene?.windows.first
window?.rootViewController?.dismiss(animated: true)

Example

struct ContentView: View {
    
    @State var isPresented = false
    
    var body: some View {
        Button(action: {
            isPresented.toggle()
        }) {
            Text("ContentView")
                .font(.title3)
                .background(.gray)
                .padding()
        }
        .fullScreenCover(isPresented: $isPresented) {
            Sample2View()
        }
    }
}

struct Sample1View: View {
    
    @State var isPresented = false
    
    var body: some View {
        Button(action: {
            isPresented.toggle()
        }) {
            Text("Sample1View")
                .font(.title3)
                .background(.black)
                .padding()
        }
        .fullScreenCover(isPresented: $isPresented) {
            Sample2View()
        }
    }
}

struct Sample2View: View {
    
    @State var isPresented = false
    
    var body: some View {
        Button(action: {
            isPresented.toggle()
        }) {
            Text("Sample2View")
                .foregroundColor(.white)
                .font(.title3)
                .background(.green)
                .padding()
        }
        .fullScreenCover(isPresented: $isPresented) {
            Sample3View()
        }
    }
}

struct Sample3View: View {
    
    @State var isPresented = false
    
    var body: some View {
        Button(action: {
            isPresented.toggle()
        }) {
            Text("Sample3View")
                .foregroundColor(.white)
                .font(.title3)
                .background(.blue)
                .padding()
        }
        .fullScreenCover(isPresented: $isPresented) {
            Sample4View()
        }
    }
}

struct Sample4View: View {
    
    var body: some View {
        Button(action: {
            let scenes = UIApplication.shared.connectedScenes
            let windowScene = scenes.first as? UIWindowScene
            let window = windowScene?.windows.first
            window?.rootViewController?.dismiss(animated: true)
        }) {
            Text("Sample4View")
                .foregroundColor(.white)
                .font(.title3)
                .background(.red)
                .padding()
        }
    }
}

以下やと、iOS15でなんたらこんたらって警告でた。

UIApplication.shared.windows.first?.rootViewController?.dismiss(animated: true)