“Single family” is a property of the building, not the people living in it. A multi-family building has separately locking units with their own kitchens. Where there are too many large houses and not enough apartments, many of the houses end up with would-be apartment dwellers living communally.

You do sometimes here about communities trying to enforce prohibitions on unrelated people living together in single family homes, but where the zoning itself is almost universal in America, such enforcement is very rare.