The vapor barrier should be placed on the 'warm side', i.e. the side where the temperature is higher (as you say). So yes which side that is (what side of the insulation) depends on whether you're mostly cooling or mostly warming your house.
There are moisture regulating sheets on the market that allow vapor to pass in summer. It is many times more expensive than a simple plastic foil, but still a drop in the construction budget bucket.
In a building that will be air conditioned, putting plastic sheeting between the insulation and drywall is a bad idea. The problem is that any moisture in the (comparatively warmer) insulation cavity will condense on the (comparatively cooler) plastic sheeting, wetting out the insulation and causing mold and rot. They also prevent the wall from drying to the inside.
It works fine in cold climates that are do not install air conditioning, as I assume is the case in Finland. In the majority of the US, even up to the Canadian border, many new houses have AC.
https://www.buildingscience.com/documents/digests/bsd-106-un...