Cyber-factories were buildings filled with cyberfication machines that were expressely used for mass conversions. On Main Universe they were build by Cyberman. On Mondas, these were also known as processing lines. The patients of the Mondasian colony ship were converted through surgery performed by the surgeon of the hospital on Floor 1056, within the Conversion Theatre. The same process was used to convert Bill Potts into the first Mondasian Cyberman on the ship. The later Cybermen transformed entire apartment blocks into Cyber-Factories before rounting up the humans of the city for mass conversion they referred as the Cyber Foundries.
A London's Cyber-factory was located in London. It was Mickey Smith hacked into Cybus Industries' mainframe from Pete's World and located the code which all of the Cybermen's emotional inhibitors used (6879760) and texted it to Rose Tyler's mobile phone. The Tenth Doctor loaded it to the computer system. The code was shut down and the inhibitors turned off. All the Cybermen realised what they had become and died from the emotions they felt. John Lumic remained, having been converted into the Cyber-Controller, only to be lost in the destruction of the factory. (TV: The Age of Steel)
Soon after, a zeppelin was sent to the Paris' Cyber-factory in Paris where it transmitted the same code. It disabled that factory's emotional inhibitor. (GAME: Save Paris) In other cases, a Preacher was sent into the factory to destroy the inhibitor with his firearm. (GAME: Cyber Assault)