How do sweet manufacturers put a coating on chocolate centered "M&Ms" or "Smarties" without melting the core?

In a large tumbler drum they tumble the chocolate pieces and slowly add the coating. As the pieces tumble, they get coated. This is the same method that drug manufacturers use to coat pills.

jonny sunday is absolutely correct.

In addition, the core contains more flour and less cacao butter. Cacao butter has a low melting point. It melts above room temperature 25°C.

on a spin dryer