Where to put the code?

I following the book "C# Tutorial" (https://video2.skills-academy.com/en-us/dotnet/csharp/tutorials/default-interface-methods-versions).
Since the book doesn't mention where the code should be placed, I just paste the code on a new tab "customer-relationship.cs".

The other codes are as follows.

ICustomer.cs

using System;  
using System.Collections.Generic;  
  
namespace customer_relationship  
{  
    // <SnippetICustomerVersion1>  
    public interface ICustomer  
    {  
        IEnumerable<IOrder> PreviousOrders { get; }  
  
        DateTime DateJoined { get; }  
        DateTime? LastOrder { get; }  
        string Name { get; }  
        IDictionary<DateTime, string> Reminders { get; }  
    }  
    // </SnippetICustomerVersion1>  
}  

IOrder.cs

using System;

namespace customer_relationship
{
// <SnippetIOrderVersion1>
public interface IOrder
{
DateTime Purchased { get; }
decimal Cost { get; }
}
// </SnippetIOrderVersion1>
}

SampleCustomer.cs

using System;
using System.Collections.Generic;

namespace customer_relationship
{
public class SampleCustomer : ICustomer
{
public SampleCustomer(string name, DateTime dateJoined) =>
(Name, DateJoined) = (name, dateJoined);

    private readonly List<IOrder> allOrders = new List<IOrder>();  

    public IEnumerable<IOrder> PreviousOrders => allOrders;  

    public DateTime DateJoined { get; }  

    public DateTime? LastOrder { get; private set; }  

    public string Name { get; }  

    private readonly Dictionary<DateTime, string> reminders = new Dictionary<DateTime, string>();  
    public IDictionary<DateTime, string> Reminders => reminders;  

    public void AddOrder(IOrder order)  
    {  
        if (order.Purchased > (LastOrder ?? DateTime.MinValue))  
            LastOrder = order.Purchased;  
        allOrders.Add(order);  
    }  
}  

}

Program.cs

using System;

namespace customer_relationship
{
// <SnippetIOrderVersion1>
public interface IOrder
{
DateTime Purchased { get; }
decimal Cost { get; }
}
// </SnippetIOrderVersion1>
}