I have been working on dummy project to understand binding in DataGridComboBoxColumn and in that I have encountered to SelectedValuePath and SelectedValue Binding.
I have looked up to various source of other questions too to understand such as follows
to understand bindings
and this,
and at last this too

the problem in my code is it gets bonded with SelectedItemBinding but not with SelectedValuePath & SelectedValueBinding

using System.Collections.ObjectModel;
using System.ComponentModel;
using System.Linq;
using System.Windows;
using System.Windows.Controls;

namespace WpfApplication10
{
    /// <summary>
    /// Interaction logic for MainWindow.xaml
    /// </summary>
    public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();
            DataContext = new MainViewModel();



        }

   
    }


    public class Item
    {
        public int ID { get; set; }
        public string Name { get; set; }
        public string Code { get; set; }

        // Property for the composite value
        public string CombinedValue => $"{ID}_{Name}_{Code}";
    }

    public class MainViewModel : INotifyPropertyChanged
    {
        

        public event PropertyChangedEventHandler PropertyChanged;
        private void OnPropertyChanged(string propertyName)
        {
            PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
        }

        private Item _selectedItem;
        public Item SelectedItem
        {
            get { return _selectedItem; }
            set
            {
                if (_selectedItem != value)
                {
                    _selectedItem = value;
                    OnPropertyChanged(nameof(SelectedItem));
                }
            }
        }

       
        

        public ObservableCollection<Item> Items { get; set; }
        public ObservableCollection<string> Codes { get; set; }

        public MainViewModel()
        {
            Items = new ObservableCollection<Item>
        {
            new Item { ID = 1, Name = "Item 1", Code = "A" },
            new Item { ID = 2, Name = "Item 2", Code = "Z" },
            new Item { ID = 3, Name = "Item 3", Code = "C" },
            new Item { ID = 4, Name = "Item 4", Code = "A C" }
        };
            Codes = new ObservableCollection<string>(Items.Select(item => item.Code).Distinct());
            SelectedItem = Items.FirstOrDefault();
        }

        
    }
}

I have tried below with SelectedItemBinding and it works

<DataGridComboBoxColumn Header="Code" SelectedItemBinding="{Binding Code}" Width="100" IsReadOnly="True">
                    <DataGridComboBoxColumn.ElementStyle>
                        <Style TargetType="ComboBox">
                            <Setter Property="ItemsSource" Value="{Binding DataContext.Codes, 
                                RelativeSource={RelativeSource AncestorType=Window}}"/>
                        </Style>
                    </DataGridComboBoxColumn.ElementStyle>
                    
                </DataGridComboBoxColumn>

but I want to implement above with SelectedValueBinding & SelectedValuePath

I have tried this till now

<DataGridComboBoxColumn Header="Value-Binded" ItemsSource="{Binding DataContext.Items}"
                                        SelectedValuePath="Code"
                                        SelectedValueBinding="{Binding ID , UpdateSourceTrigger=PropertyChanged}"
                                        >                   
                </DataGridComboBoxColumn>

DataGrid Defined as

 <DataGrid ItemsSource="{Binding Items}" AutoGenerateColumns="False" CanUserAddRows="False">

And DataContext as

<Window.DataContext>
        <local:MainViewModel/>
    </Window.DataContext>