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The bigger cold war era civil defense bunkers around here (Czech Republic) usually have the following design for ventilation: * one or two filtration rooms, powered by electricity with optional manual backup * one or two diesel generators to provide electricity for the filtration system, lighting and other equipment * high pressure oxygen bottles for couple hours of total isolation - intended for the first few hours after a nuclear detonation when the regular filtration equpment could be overwhelmed by short halflife fallout and smoke/lack of oxygen due to the firestorm of the city arround burning

Examples of such shelters: - general purpose shleter Denis (capacity op to 2000 people): https://podzemibrno.cz/en/places-underground/cover-denis-und... - army headquarters shelter in the Vypustek cave (capacity 100+ people): https://vypustek.caves.cz/en - 10-Z shelter, Brno area civil defense headquarters (capacity 100+ people): https://10-z.cz/en/

Smaller shelters that could be found under many 50s era building were much more rudimentary, usually without idepedent source of electricity and just simple hand cranked filtration system.