public class Order
{
    public int OrderId { get; set; }
    public string OrderNumber { get; set; }
    public Customer Customer { get; set; }
}

public class Customer
{
    public int CustomerId { get; set; }
    public string Name { get; set; }
    public List<Order> Orders { get; set; }
}

public class MyDbContext : DbContext
{
    public DbSet<Order> Orders { get; set; }
    public DbSet<Customer> Customers { get; set; }

    protected override void OnModelCreating(ModelBuilder modelBuilder)
    {
        modelBuilder.Entity<Order>(entity =>
        {
            entity.HasOne(o => o.Customer)
                .WithMany(c => c.Orders)
                .HasForeignKey(o => o.CustomerId);
        });
    }
}

var order = dbContext.Orders.FirstOrDefault();
var customerName = order.Customer.Name;