There is a How It’s Made episode showing how Kraft singles are made somewhere. It’s still cheese, but they also add more milk to make it meltier, as well as things like preservatives. It’s kind of like a solidified bechamel.

Other brands of similar processed cheese slices tend to made entirely with oils with zero dairy. There is a definite difference in taste and texture comparing Kraft Singles, off-brand singles, and just plain cheddar though.