how does the open door fridge in stores retain chillness compared to the 1one we have at home?

Answer 1

This is, done by running it all the time and having large enough capacity, most are open at the top so the cold air is retain inside the show case and surrounding air is very cold also.

If you are energy minded then for home you should have chest freezer rather then up right freezer. Upright freezer is fairly recent appliance, for home upright were almost unknown in 1960's.

Answer 2

I guess it is because the fridges used in stores are for professional use only.

Answer 3

usually the stores own a fridge with higher cooling capacity (in terms of tonnage) than in our domestic units.

Answer 4

They are bigger units.

Just to keep the milk cool enough to keep it good until it's sold, the shelving the milk sits on is quite cold.