Alexander understood this very well. Master craftsmen trained their apprentices for years, until they became journeymen – that's when they could actually contract themselves out for hire. But a master craftsman always oversaw groups of builders.
Through strict conformance to tradition, window-makers knew that *this* was the way to build a window; bricklayers knew that *these* were the ways in which bricks could be laid, etc.
Their forms and patterns had evolved over time, and the end result of their combination was environments and cities of great coherence and functionality.