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
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.
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.