SwiftUI AppStorage, UserDefaults and ObservableObject

You're creating instances of AppData in multiple places. In order to share data, you have to share one instance of AppData.

I'm presuming that you create AppData in a parent view of ContentView since you have @ObservedObject var appData: AppData defined at the top of the view (without = AppData()). This is probably in your WindowGroup where you also must have a NavigationView.

I removed the next (let createpage = CreatePage(appData: AppData())) because it does nothing. And in the NavigationLink, I passed the same instance of AppData.

struct ContentView: View {
    // MARK: - PROPERTY
    @StateObject var appData: AppData = AppData() //Don't need to have `= AppData()` if you already create it in a parent view
    
    var body: some View {
        // I'm assuming there's a NavigationView in a parent view
        VStack { //note that I've wrapped the whole view in a VStack to avoid having two root nodes (which can perform differently in NavigationView depending on the platform)
            HStack {
                NavigationLink("+ create a shortcut", destination: CreatePage(appData: appData))
                    .padding()
                Spacer()
            } //: HStack - link to creat page
            VStack {
                Text("\(appData.shortcutTitle) - \(appData.shortcutOption)")
            }
        }
    }
}

struct CreatePage: View {
    // MARK: - PROPERTY
    @AppStorage("title") var currentShortcutTitle: String?
    @AppStorage("option") var currentOption: String?
    @Environment(\.presentationMode) var presentationMode
    @ObservedObject var appData: AppData
    
    var body: some View {
        NavigationView{
            ScrollView{
                Text("Create a ShortCut")
                    .padding()
                
                HStack {
                    TextField("what is the title?", text: $appData.titleInput)
                        .textFieldStyle(RoundedBorderTextFieldStyle())
                        //.frame(width: 150, height: 60, alignment: .center)
                        .border(Color.black)
                        .padding()
                } //: HStack - Textfield - title
                .padding()
                
                HStack (spacing: 10) {
                    TextField("options?", text: $appData.optionInput)
                        .textFieldStyle(RoundedBorderTextFieldStyle())
                        .frame(width: 80, height: 40, alignment: .leading)
                        .padding()
                } //: HStack - Textfield - option
                .padding()
                
                Button(action: {
                    appData.shortcutTitle = appData.titleInput
                    appData.shortcutOption = appData.optionInput
                    presentationMode.wrappedValue.dismiss()
                }, label: {
                    Text("Save")
                        .padding()
                        .frame(width: 120, height: 80)
                        .border(Color.black)
                }) //: Button - save
                .padding(.top, 150)
            } //: Scroll View
        } //: Navigation View
    } //: Body
}

Regarding @AppStorage and UserDefaults, it's a little hard to tell what your intent is at this point with those. But, you shouldn't need to declare AppStorage and call UserDefaults on the same key -- @AppStorage writes to UserDefaults for you. Read more at https://www.hackingwithswift.com/quick-start/swiftui/what-is-the-appstorage-property-wrapper

you can use a singleton ObservableObject that conforms to NSObject so you can observe everything even older apple objects like progress.

class appData : NSObject , ObservableObject {
    static let shared = appData()
    @Published var localItems = Array<AVPlayerItem>()
    @Published var fractionCompleted : Double = 0
    @Published var downloaded : Bool = false
    @Published var langdentifier = UserDefaults.standard.value(forKey: "lang") as? String ?? "en" {
        didSet {
            print("AppState isLoggedIn: \(langIdentifier)")
        }
    }
    var progress : Progress?
    override init() {
    }
}

then u can use it anywhere in your code like this:

appData.shared.langIdentifier == "en" ? .leading : .trailing

You should be able to simply put AppStorage objects in the ObservableClass and call them from there. There should be no need to put AppStorage in the View and then read from UserDefaults in the class.

class AppData: ObservableObject {
    @AppStorage(keyForValue) var valueForKey: ValueType = defaultValue
    ...
}

Of course, you could make @Published property in the class and define the getter and setter for it so it reads and writes directly to the UserDefaults but at that point, you're just creating more work than simply using AppStorage from the beginning directly in the class.