A mirror behaves as a mirror because on one side of the glass we coat it with something reflective. This is usually at the back of the mirror. Most of the time it is aluminium or silver.
The second reason, is me thinking that perhaps you’ve noticed that a lot of glass mirrors look greenish from the side. This is to do with the quality of the glass used to make it. Most ordinary mirrors use glass where we don’t really care about the transmission (how much light gets through). This means it can have lots of defects and impurities (stuff that shouldn’t be in the mix of silicon dioxide, which is in sand, we melt to make glassy structures) in the glass which will cause different colours to be absorbed. With a window, we care more about letting lots of light in so the transmission of light becomes more important, so we’ll put more effort in to making sure there are less impurities. This is why the colour of mirror glass might look different from the side compared to that in a window.