At the paper mill, cartons are added to a large machine called a Hydrapulper (imagine a giant blender) that uses water to break the cartons down into their component parts. The fiber is separated from the plastic and aluminum.
Alternatively, at a whole carton recycler, cartons are shredded into a million pieces. Then heat is applied and the shredded pieces are pressed back together into large sheets (like a panini press made of shredded cartons!). About 30 cartons can turn into a single 2’x2’ ceiling tile, while roughly 400 cartons make up each wallboard.