creating a object during runtime

I want to create a camera in a game/scene during runtime using c# script. is that possible without having an existing reference object pre-made in the scene, or a prefab in the files?

Yes. You can simply create new GameObjects using the constructor of the GameObject class:

var obj = new GameObject();

That object will have a Transform component but nothing else. You can add other components using AddComponent:

var camera = obj.AddComponent();