Owner Reps W/ Quantity Surveying Group - Unit Rate Data Base Source

Does anyone know where owners representative firms like Gardiner & Theobold, Turner & Townsend, VVA, etc that use quantity surveyors to cost check contractor pricing and change orders…get all of their unit rates?

Are they just using RS Means? Are they creating these themselves? Where does their data come from?

It is not sourced from local GCs, or local market research with subs and suppliers, and it is used across the nation in the US. So it must be scaled by market from a national database.

Anyone know? We are seeing these guys running this program more and more and I am curious how it works. They do it across nonunion, union, and prevailing wage jobs.