Builder pattern in C#: Test Data Builder for xUnit tests of a domain aggregate

// ======================================================================
// Q: Builder pattern: when do you use it?
//
// When an object has many optional parameters or is built in steps. In .NET:
// WebApplication.CreateBuilder(), StringBuilder, UriBuilder. In practice I use
// it a lot in tests (Test Data Builder), like the OrderBuilder from Q20.
// In the interview don't just name patterns. Say what problem it solved and when
// you don't use it. A pattern without a reason to exist is over-engineering.
// ======================================================================

// Test Data Builder
public class OrderBuilder
{
    private int _customerId = 1;
    private readonly List<(string Sku, int Qty)> _lines = [];

    public static OrderBuilder Draft() => new();
    public OrderBuilder ForCustomer(int id)            { _customerId = id; return this; }
    public OrderBuilder WithLine(string sku, int qty)  { _lines.Add((sku, qty)); return this; }

    public Order Build()
    {
        var order = Order.Create(_customerId);
        foreach (var (sku, qty) in _lines)
            order.AddLine(sku, qty, new Money(10m, "EUR"));
        return order;                                  // always valid, goes through the domain rules
    }
}