I think one of the better explanations is to view production along the entire supply chain as the production of the commodity, not just in the moment of a factory. Socially necessary functions all require socially necessary labor, and this amalgum of socially necessary labor and raw materials forms the commodity. A commodity is not just a commodity in itself, it is a commodity that has been transported, advertised, and sold. It does get more complicated to calculate, but you can also break it down into its constituent elements.