Zbrush to Unity3d

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I'm new to 3d. I watched some tutorials for Zbrush. I want to make a model low poly and import the low divisions (low poly) to Unity3d. As seen on pic, is it possible to import to Unity3d the (A) model, and make it look like (B) without adding polygons?
What does normal map do exactly? What does displacement do, and where do I put it in Unity3d's standard shader?