If we pointed our telescopes at a galaxy located 1 million light years away and that galaxy had a giant mirror pointing back at us, we would see our own galaxy as it was 1 million years ago.
When we observe distant objects in space, we are actually observing them as they were in the past because it takes time for light to travel through space. Light travels at a finite speed, and in the case of the distance you mentioned, it would take approximately 1 million years for light from that galaxy to reach us.
So, if the distant galaxy had a giant mirror that reflected light back towards us, the light would still need to travel 1 million light years to reach our telescopes. By the time we receive that light and observe it, we would be seeing the light that was originally emitted from our own galaxy 1 million years ago.
In other words, the light we would observe in our telescopes would be a snapshot of our galaxy's past, showing it as it appeared 1 million years ago, rather than its current state.