Generally for street we'd use plats, but unplatted streets (like FM 2499 extension or Lewisville Lake Toll bridge, etc) we just have to know about. At Rand McNally we outsourced data collection, at Mapsco we just got a lot of newspapers and drove around a lot. You have to call cities for city limit changes and ISDs for new schools.
As for the incorrect US Highway numbers they probably just had a source and without anything better they copied that source. Thats a cool map, btw.