C# Online Compiler | .NET Fiddle

using System;
using System.Collections.Generic;
using System.Linq;
					
public class Program
{
	public static void Main()
	{
		List<Customer> customers = new List<Customer>
		{
			new Customer { CustomerID = 1, Name = "Alice", Country = "USA" },
			new Customer { CustomerID = 2, Name = "Bob", Country = "Canada" },
			new Customer { CustomerID = 3, Name = "Charlie", Country = "UK" },
			new Customer { CustomerID = 4, Name = "David", Country = "USA" },
			new Customer { CustomerID = 5, Name = "Eve", Country = "Canada" }
		};

		List<Order> orders = new List<Order>
		{
			new Order { OrderID = 1, CustomerID = 1, OrderDate = DateTime.Today.AddDays(-10), ProductIDs = new List<int> { 1, 2 } },
			new Order { OrderID = 2, CustomerID = 2, OrderDate = DateTime.Today.AddDays(-15), ProductIDs = new List<int> { 3, 4 } },
			new Order { OrderID = 3, CustomerID = 3, OrderDate = DateTime.Today.AddDays(-20), ProductIDs = new List<int> { 1, 4 } },
			new Order { OrderID = 4, CustomerID = 4, OrderDate = DateTime.Today.AddDays(-25), ProductIDs = new List<int> { 2, 3 } },
			new Order { OrderID = 5, CustomerID = 5, OrderDate = DateTime.Today.AddDays(-30), ProductIDs = new List<int> { 1, 5 } }
		};

		List<Product> products = new List<Product>
		{
			new Product { ProductID = 1, Name = "Laptop", Price = 1000.00m },
			new Product { ProductID = 2, Name = "Smartphone", Price = 500.00m },
			new Product { ProductID = 3, Name = "Tablet", Price = 600.00m },
			new Product { ProductID = 4, Name = "Monitor", Price = 300.00m },
			new Product { ProductID = 5, Name = "Keyboard", Price = 50.00m }
		};

		
		// Assuming the existence of lists: customers, orders, products

		// Step 1: Calculate the total spending per customer
		var customerSpendings = orders
			.GroupBy(o => o.CustomerID)
			.Select(g => new
			{
				CustomerID = g.Key,
				TotalSpending = g.SelectMany(o => o.ProductIDs)
								 .Join(products, pid => pid, p => p.ProductID, (pid, p) => p.Price)
								 .Sum()
			})
			.Join(customers, sp => sp.CustomerID, c => c.CustomerID, (sp, c) => new { c.Name, sp.TotalSpending })
			.ToList();

		// Step 2: For each country, find the customer who has spent the most
		var topSpendersByCountry = customerSpendings
			.Join(customers, sp => sp.Name, c => c.Name, (sp, c) => new { c.Name, c.Country, sp.TotalSpending })
			.GroupBy(x => x.Country)
			.Select(g => new
			{
				Country = g.Key,
				TopSpender = g.OrderByDescending(x => x.TotalSpending).First().Name,
				MaxSpending = g.Max(x => x.TotalSpending)
			})
			.ToList();

		// Step 3: Identify the most popular product
		var mostPopularProduct = orders
			.SelectMany(o => o.ProductIDs)
			.GroupBy(pid => pid)
			.Select(g => new { ProductID = g.Key, CustomerCount = g.Count() })
			.Join(products, gp => gp.ProductID, p => p.ProductID, (gp, p) => new { p.Name, gp.CustomerCount })
			.OrderByDescending(x => x.CustomerCount)
			.FirstOrDefault();

		// Output the results
		Console.WriteLine("Total spending per customer:");
		foreach (var sp in customerSpendings)
		{
			Console.WriteLine($"{sp.Name}: {sp.TotalSpending}");
		}

		Console.WriteLine("\nTop spenders by country:");
		foreach (var top in topSpendersByCountry)
		{
			Console.WriteLine($"{top.Country}: {top.TopSpender} ({top.MaxSpending})");
		}

		Console.WriteLine($"\nMost popular product: {mostPopularProduct.Name} (Ordered by {mostPopularProduct.CustomerCount} customers)");

		
	}
}

public class Customer
{
    public int CustomerID { get; set; }
    public string Name { get; set; }
    public string Country { get; set; }
}

public class Order
{
    public int OrderID { get; set; }
    public int CustomerID { get; set; }
    public DateTime OrderDate { get; set; }
    public List<int> ProductIDs { get; set; }
}

public class Product
{
    public int ProductID { get; set; }
    public string Name { get; set; }
    public decimal Price { get; set; }
}

        Console.WriteLine($"\nMost popular product: {mostPopularProduct.Name} (Ordered by {mostPopularProduct.CustomerCount} customers)");
 
using System;
using System.Collections.Generic;
using System.Linq;
                    
public class Program
{
    public static void Main()
    {
        List<Customer> customers = new List<Customer>
        {
            new Customer { CustomerID = 1, Name = "Alice", Country = "USA" },
            new Customer { CustomerID = 2, Name = "Bob", Country = "Canada" },
            new Customer { CustomerID = 3, Name = "Charlie", Country = "UK" },
            new Customer { CustomerID = 4, Name = "David", Country = "USA" },
            new Customer { CustomerID = 5, Name = "Eve", Country = "Canada" }
        };
​
        List<Order> orders = new List<Order>
        {
            new Order { OrderID = 1, CustomerID = 1, OrderDate = DateTime.Today.AddDays(-10), ProductIDs = new List<int> { 1, 2 } },
            new Order { OrderID = 2, CustomerID = 2, OrderDate = DateTime.Today.AddDays(-15), ProductIDs = new List<int> { 3, 4 } },
            new Order { OrderID = 3, CustomerID = 3, OrderDate = DateTime.Today.AddDays(-20), ProductIDs = new List<int> { 1, 4 } },
            new Order { OrderID = 4, CustomerID = 4, OrderDate = DateTime.Today.AddDays(-25), ProductIDs = new List<int> { 2, 3 } },
            new Order { OrderID = 5, CustomerID = 5, OrderDate = DateTime.Today.AddDays(-30), ProductIDs = new List<int> { 1, 5 } }

Cached Result