I didn't assign a value to the instance in singleton, but it is assigned automatically.

I figured it out by outputting Debug.Log() several times.

obj.AddComponent();

In that part, the instance is automatically assigned a value.

Any idea why the value is automatically assigned to instance?

 public static GameInfo Instance   {
     get
     {
       if (instance == null)
       {
         instance = FindObjectOfType<GameInfo>();
         if (instance == null)
         {
           GameObject obj = new GameObject();
           obj.name = "GameInfo";
           obj.AddComponent<GameInfo>();
         }
       }
       return instance;
     } }