These shelters were built into the basement system, often extending outside of the building, underground. Air raid drill alarms were conducted semi regularly, with people expected to consider the route of evacuation from their apartments. The shelters were basically concrete corridors and rooms. Ventilation was provided by on-the-ground structures, such as the one shown in the photo. You can see these in numerous neighborhoods around Warsaw, and if you're lucky you may be able to visit one.
A vent to a bomb shelter serving three apartment buildings: