A minyan factory is a shul that does not NECESSARILY serve a perticular Kehilla, or even if it officialy does, due to it’s facilities (i.e. mikva etc), size or location, it serves all members of that city/town etc.
More than anything though, usually, is the sheer amount of minyanim happening in that one shul, many times not having a strict timing as in start times for minyanim, rather one starting as soon as the last finishes.
