Simplest answer to the first question (from a non-expert): mammals that regularly live in the open cold tend to have specialized fur coats. These are really good insulators. That and their metabolism probably works in favour of keeping them warm.

As to the second question, hunter gatherer societies can use the pelts of the animals they hunt, as well as build structures that minimize the amount of wind you have to deal with. Often involving animal hides again, or other kinds of shelter. If you've ever been inside a hollowed out structure of snow (i.e. a quinzee), it is relatively warm compared to having no shelter at all.