DontDestroyOnLoad Question

Hello.

I have my Game Controller set up as a singleton, however when transitioning between scenes some of the data is lost. I was under the assumption that if I use DontDestroyOnLoad() it would prevent data loss between scenes. Do I have to use this for each object I don't want destroyed and not on a single object containing other objects?

Also, if my singleton, which is static, isn't in the scene (in the editor) will it still work as expected if I use DontDestroyOnLoad() because I keep getting errors that certain GameObjects I expect to be loaded were destroyed during the scene change.

If DontDestroyOnLoad doesn't keep an entire object loaded, then what is it for? Or am I not understanding how to use it right?

For instance:

 public class GameController : MonoBehaviour
 {
     // Game Control Objects
     public static GameController GC;

     // Does this stuff not stay loaded?
     GameObject menu1, menu2; //....etc
 
     void Awake()
     {
          if(GC == null)
         {
             DontDestroyOnLoad(GC);
             GC = this;
         }    
      }
 }

I appreciate any help. Thanks for reading.