C# Online Compiler | .NET Fiddle

using System;
using System.Collections.Generic;
using System.Linq;

public class Program
{
    public static void Main()
    {
        List<Product> products = new List<Product>
        {
            new Product { ProductID = 1, Name = "Laptop", IsBackordered = true },
            new Product { ProductID = 2, Name = "Tablet", IsBackordered = false },
            new Product { ProductID = 3, Name = "Smartphone", IsBackordered = true }
        };

        List<OrderDetail> orderDetails = new List<OrderDetail>
        {
            new OrderDetail { OrderDetailID = 1, OrderID = 1, ProductID = 1, Amount = 1000m },
            new OrderDetail { OrderDetailID = 2, OrderID = 1, ProductID = 3, Amount = 500m },
            new OrderDetail { OrderDetailID = 3, OrderID = 2, ProductID = 1, Amount = 1000m },
            new OrderDetail { OrderDetailID = 4, OrderID = 2, ProductID = 2, Amount = 700m }
        };

        List<Order> orders = new List<Order>
        {
            new Order { OrderID = 1, CustomerID = 1, OrderDate = new DateTime(2023, 3, 10), OrderDetails = orderDetails.Where(od => od.OrderID == 1).ToList() },
            new Order { OrderID = 2, CustomerID = 2, OrderDate = new DateTime(2023, 3, 11), OrderDetails = orderDetails.Where(od => od.OrderID == 2).ToList() }
        };

        List<Customer> customers = new List<Customer>
        {
            new Customer { CustomerID = 1, Name = "Alice" },
            new Customer { CustomerID = 2, Name = "Bob" }
        };

        var customerSpendingOnBackorders = orders
            .SelectMany(o => o.OrderDetails, (o, od) => new { o.CustomerID, od.ProductID, od.Amount })
            .Join(products.Where(p => p.IsBackordered), o => o.ProductID, p => p.ProductID, (o, p) => new { o.CustomerID, o.Amount })
            .GroupBy(x => x.CustomerID)
            .Select(g => new
            {
                CustomerID = g.Key,
                TotalSpending = g.Sum(x => x.Amount)
            })
            .Join(customers, c => c.CustomerID, cu => cu.CustomerID, (c, cu) => new { cu.Name, c.TotalSpending })
            .ToList();

        var productBackorderInfo = orders
            .SelectMany(o => o.OrderDetails)
            .Join(products.Where(p => p.IsBackordered), od => od.ProductID, p => p.ProductID, (od, p) => new { p.ProductID, p.Name, od.Amount })
            .GroupBy(x => x.ProductID)
            .Select(g => new
            {
                ProductID = g.Key,
                Frequency = g.Count(),
                TotalSales = g.Sum(x => x.Amount),
                ProductName = g.First().Name
            })
            .OrderByDescending(x => x.Frequency)
            .First();

        Console.WriteLine("Customers and Their Spending on Backordered Products:");
        foreach (var item in customerSpendingOnBackorders)
        {
            Console.WriteLine($"Customer: {item.Name}, Total Spending: {item.TotalSpending}");
        }

        Console.WriteLine($"\nProduct with Highest Frequency of Backorders: {productBackorderInfo.ProductName}, Total Sales: {productBackorderInfo.TotalSales}");
    }
}

public class Product
{
    public int ProductID { get; set; }
    public string Name { get; set; }
    public bool IsBackordered { get; set; }
}

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

public class OrderDetail
{
    public int OrderDetailID { get; set; }
    public int OrderID { get; set; }
    public int ProductID { get; set; }
    public decimal Amount { get; set; }
}

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

        Console.WriteLine($"\nProduct with Highest Frequency of Backorders: {productBackorderInfo.ProductName}, Total Sales: {productBackorderInfo.TotalSales}");
 
using System;
using System.Collections.Generic;
using System.Linq;
​
public class Program
{
    public static void Main()
    {
        List<Product> products = new List<Product>
        {
            new Product { ProductID = 1, Name = "Laptop", IsBackordered = true },
            new Product { ProductID = 2, Name = "Tablet", IsBackordered = false },
            new Product { ProductID = 3, Name = "Smartphone", IsBackordered = true }
        };
​
        List<OrderDetail> orderDetails = new List<OrderDetail>
        {
            new OrderDetail { OrderDetailID = 1, OrderID = 1, ProductID = 1, Amount = 1000m },
            new OrderDetail { OrderDetailID = 2, OrderID = 1, ProductID = 3, Amount = 500m },
            new OrderDetail { OrderDetailID = 3, OrderID = 2, ProductID = 1, Amount = 1000m },
            new OrderDetail { OrderDetailID = 4, OrderID = 2, ProductID = 2, Amount = 700m }
        };
​
        List<Order> orders = new List<Order>

Cached Result

<?xml version="1.0" encoding="utf-16"?>
<Namespace xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
  <Types>
    <Type Name="name" />
  </Types>
</Namespace>