Pretty much what the other guy said. In climates where temps get below freezing, you have to keep your thermostat at 45°F minimum (though most do 55), and pipes are generally run above the insulation in the floor joist. Pipes are run in such a way that they are sloped down towards the stop and waste valve (basically a valve below the frost line that cuts off the water supply into the house and also drains the water inside the pipes when shut off once you open all the faucets). Even lines out of a central water heater need to be considered since unless a circ pump is installed and always running, they’ll freeze the same as cold pipes.