Question

When the only code in there is for a class how would you code it adding public to the default class like so

namespace HW2_2_Spaceship
{
   public class Spaceship //added public to the default class 
    {
      static void Main(string[] args)
        {
        }

or

namespace HW2_1_Book
{
    class Book
    {
        static void Main(string[] args)
        {

        }
        public class Book // added a new class with in the default class
        {

Answer 1

In general, each class should have their own file.

Main should be in Program.cs

There are usecases where you can use Inner classes, see Using Inner classes in C#.