Yeah you usually do mirrors by rendering the scene from the mirrors perspective.

Think of the mirror as a plane.

Now, to render whatever the mirror should see, simply REFLECT MIRROR the real camera to the other side of the mirror and render (render only the area of screen covered by mirror) on top of the real scene. The mirrored camera should not render anything behind the mirror plane (so you only render the part of scene on one side of the mirror, the side that can actually be reflected off of it)

Idk how to do that in unity tho.

I have recently implemented this technique using my own engine (a clone of the Unity API so should be quite similar) and it basically involved the following steps (There may be a better way to do this but this is simple):

1) add another camera just in front of the mirror facing outwards.

2) Set up this camera to render to a RenderTexture.

3) Set the mainTexture of the mirror's material to the same RenderTexture that you assigned to the mirror camera.

4) (Optional): Set the mirror's layer so that the mirror camera does not render it. Though it will probably just come out black anyway since a render texture cannot render itself to itself (if that makes sense). Luckily it is facing the other way so it wont make much difference.