I did notice that if I assign scripts that generate singleton classes (for my managers), to an empty game object, everything works fine.

Now I would like to start the scene with the empty game object, attach the main game manager singleton, and in the game manager singleton I would like to attach to it the other manager singleton classes.

Although I am not sure how you actually do so. Do you run addComponent to the main game manager, and they will be attached to the existing gameobject?

The gameobject that I use for the main gamemanager is set to not be destroyed on load; the doubt is if I should run addComponent on the scripts, or create a reference from the gamemanager script and instantiate it from there.

I did look for examples on the subject, but the only thing that I found is the reference to add a script to a game object; and I am not sure if that's actually what is supposed to be done.