Answer:
A thermos keeps cold things cold in the same way. It doesn't contain some kind of cooling device. The same vacuum that keeps hot things hot keeps cold things cold. Heat that might otherwise transfer to the cold contents of the thermos is prevented from reaching it because of the vacuum between the thermos walls.
Today's thermoses are constructed much more sturdily than those in the past. The first thermoses featured metal exteriors with glass interior walls. These thermoses often ended up getting broken when accidentally dropped.