Mirrors are a particularly interesting part of magic. They're gateways for otherwordly entities, cause bad luck when broken, and most importantly...
There is a scientific explanation for all of that.
Now, as far as the otherwordly creatures (i.e: demons) go, they're malicious AIs that hijack naturally occurring (don't ask) utility fogs and use them to torment people who unironically do 3 AM challenges.
Utility fogs are just a swarm of tiny machines that can grab each other and together, exert force on objects. There's more to them, but you should check the link if you want to read about that.
But how do mirrors come into this?
In paper, there are certain demons that need a mirror to manifest. The mirror has to be made out of a metal and has to be able to reflect (so a VERY dirty mirror won't do). Once the ritual is complete, the demon briefly appears in the mirror and begins the free-roam phase, which is for another question.
But what's the physics-related reason for these demons to require mirrors to manifest?