They're just a solidified cheese beshmel. It's cheddar, milk and binders to keep it solid. At least the good quality singles. Some of the cheaper ones are made with oils and have no actual cheese. Those ones behave and taste like plastic.
One time we bought a big bag of preshredded mozzarella from one of the big box stores, so we had like five pounds of it. We made lasagna. Instead of melting, it browned. Crisped. We looked closer. It was some imitation cheese.
Gotta say tho, lasagna with some of the cheese crispy and some melted? That was a good week
Its actually just watered down cheese with like two other ingredients which are common food additives. This is a great vid of someone making their own 'american cheese'