Lets just say the data intersection of: density and geocoding of users, density and location of office sites, travel time bands, and service level expectations all have a part in the final calculation.
The data is either base layers imported, or calculated layers. There is no one "comprehensive layer" nor does that concept make sense. Different groups handle their own data. We import it in and use their data for our final result.
(In fact, its the OSM approach of everything as 1 layer and tags everywhere is why a DB import and data handling is so obnoxious.)