Does Ready allows games to be for commercial use like putting ads?
You could make ads with gifs and sounds.
Try and find a company that will pay you to advertise their products in a Ready game.
Yes, you can embed advertising after export to the Unity, for example from Admob service. We have nothing against it.
Is this only possible in Unity? (uploading ready game to unity)
@franzsian - yes, now only in the Unity.