How does a master key work?

The super of our apartment building is always going into everyone's apartment when something needs to be fixed. There must be a hundred apartments, but he only carries one key around with him. How does he get into all those apartments?

How Does a Master Key Work? - USA Lock & Key Experts

What is a Master Keyed System?

If keys unlock things based on their shape, how do master keys

Master Key System in Factories Types, Levels & How Does It Work?

ELI5: How does a master key work? : r/explainlikeimfive

How Does a Master Key Work on Various Doors?

If keys unlock things based on their shape, how do master keys

How Does A Master Key Work? - Guardian Safe & Lock LLC

Do You Have Control of Your Security Keys? A Q&A on Master Key

How SECURE is your nice new house

Orange County Commercial Locksmith and Master Key Systems

ELI5: How does a master key work? : r/explainlikeimfive

Lock Surgeon Master Key Systems Locksmith

How to make a Kwikset master key - Quora