Resin bonds to the surface it's poured on in one of two ways. Chemical bond, which is when you add a second layer of resin to a layer of resin that isn't completely cured. And mechanical, which means you pour resin on a surface that has some 'grip'. Either naturally like wood or by sanding to create a texture.
So you can pour over wood and it will bond. Epoxy and glass do not contract and expand at the same rate so the glass could crack at some point if it's exposed to extreme temperature changes. When you pour over glass or mirrors be sure it is super clean and lightly sand the edges to give it some 'tooth' for the resin to bond to.