Instantiate 30 seconds before GameStarts?

Actually, Unity manages pretty much all the instantiation stuff for you. However, Unity also provides all means to handle stuff yourself if you wish to do so.

For example, you could start instantiating objects in your startup scene which then persist when you load your actual "game scene".

To keep the objects when loading a new scene, use Object.DontDestroyOnLoad(Object target)

Another method you might be interested in: Application.LoadLevelAdditiveAsync(string levelName). You could use this to load the actual game level in the background, and then do some camera switching, for instance, to actually "start" the game.

Of course, as mentioned before, you'll have to do a lot of things that Unity usually does for your "automatically, under the hood" yourself. And how exactly you'd do that depends largely on your own needs and creativity.