they don’t really, at least not in any universal sense. in the military (marine corps) we always used YMD with slashes. i would say that in non-military contexts, dashes are seen with YMD more frequently. but dashes or slashes are used for both YMD and MDY and there's no legal rule or regulation nor requirement to use one or the other. some people just pattern match quickly and assume too much based on that.

Do they? I think the US use MDY mostly and sometimes YMD.