How to structure data in Firestore using swift

Question

I'm creating a customer management app for iOS. I'm using Firebase to store data and files. My app have multiple view, in main view are the customers and the second for saving documents and display and the third is where you can add some callDetails what you speak last time with this customer.

So far I can save data in Firestore and files in storage and display them.

My problem is how can I structure the database that each document point to the right customer and same for callDetails. I know that I need to add references in the customer collection like Nested Data for example but I'm stuck with this. So how can I save a reference to customer collection and then fetch the data?

I refer to this article but no luck: https://peterfriese.dev/posts/firestore-codable-the-comprehensive-guide/

CustomerStruct.swift

struct Customers: Identifiable, Codable {
    @DocumentID var id: String?
    var fullName: String
    var phoneNum: String
    var email: String
    var address: String
    var profession: String
    var age: String
    var dateOfBirth: String

    enum CodingKyes: String, CodingKey {
        case id
        case fullName
        case phoneNum
        case email
        case address
        case profession
        case age
        case dateOfBirth
    }
}

LastCallStruct.swift

struct LastCallData: Identifiable, Codable {
    @DocumentID var id: String?
    var title: String
    var description: String
    var date: String

    enum CodingKyes: String, CodingKey {
        //case id
        case title
        case description
        case date
    }
}

this is how i save the customers data the same is for callDetails

let newCustomers = Customers(
    fullName: nameTextField.text!,
    phoneNum: phoneTextfiled.text!, 
    email: emailTextField.text!,
    address: addressTextField.text!, 
    profession: professionTextField.text!, 
    age: ageTextField.text!, 
    dateOfBirth: dateOfBirthTexField.text!)

do {
    try self.db.collection("...Customers").addDocument(from: newCustomers)
} catch let error {
    print("Error writing data to firestore,\(error)")
}

This is how I fetch customers and the same for callDetails

func loadCustomers(){
    db.collection(self.collectionName).addSnapshotListener { (QuerySnapshot, error) in
        self.customerArray = []
        if let error = error{
            print("Error geting documents,\(error)")
        } else {
            for document in QuerySnapshot!.documents{
                print("\(document.documentID) => \(document.data())")
                let data = document.data()
                if let customerName = data["fullName"] as? String, let customerNumber =
                    data["phoneNum"] as? String {
                    let newCustomer = Customers(id: document.documentID,fullName: customerName, phoneNum: customerNumber, email: "", address: "", profession: "", age: "", dateOfBirth: "")
                    self.customerArray.append(newCustomer)
                    
                    DispatchQueue.main.async {
                        self.tableView.reloadData()
                    }
                }
            }
        }
        
    }
}

This is how i save document data in storage and put a reference in a new collection but i want this to point to customer collection

private func handleFileSelection(inURL:URL) -> Void {
    // let filePath = inURL.deletingPathExtension().lastPathComponent
    filePath = inURL.pathComponents.last!
    // file Storage reference path
    let path = "customer name/\(filePath)"
    let files = storage.reference().child(path)
    do {
        let data = try Data(contentsOf: inURL)//Geting file here
        //Upload the data for the specific file in firebase
        files.putData(data, metadata: nil) { (metadata, error) in
            //Save a reference into firestore

            self.db.collection("Documents").document().updateData(["url": path])
            guard metadata != nil else {
                print("Error while uploading data, \(String(describing: error))")
                return
            }

            files.downloadURL { (url, error) in
                //Download the file URL
                let urlString = url?.absoluteString
                print("Dowloaded URL: \(String(describing: urlString))")
            }
            print("Success")
        }
    } catch {
        print("Document loading error")
    }             
}

This is how my database looks like