I think it’s more than being a reference book, but really understanding at an intuitive level how things work and being able to explain it. They are facile with

I think it’s more than being a reference book, but really understanding at an intuitive level how things work and being able to explain it. They are facile with the concepts to such a degree they don’t have to think about how things fit together to use a system and language and sdlc, they are able to just focus on the specific challenge. Further they can unblock everyone around them by providing that intuition on demand, and often explaining concepts others struggle with in a way that gives them a similar intuition.