Where do houseflies go in the winter if it's still warm inside?

Answer 1

well, some of those houseflies may be found on the walls standing without any movement , or standing still on any other surface without being noticed.

The method of over wintering is not well understood but in some areas populations develop in doors throughout the winter.
Housefly eggs are laid in almost any warm moist material which will furnish suitable food for the growing larvae.
Animal manure,human excrement garbage,decaying.Vegetable material and ground contaminated with organic matter are suitable materials.

Answer 2

Flies have a short life span and maggots are their "babies" .
So unless you have an indoor maggot source there will be no flies in winter!

Answer 3

They die