using System is the basic framework that needs to be uploaded to do thing like
Console.write(). With out it, you’d have to do
In your second example, you are just importing other framework that you might need depending on what you are developing. From the looks of it, it is a basic game framework set: audio, graphics, touch, etc.
using is just a way to import frameworks that you might need. See: https://msdn.microsoft.com/en-us/library/zhdeatwt.aspx
solved Creating a Class in C#, which file should I use?