A commodity remains a commodity up to the point when it is consumed. Commodities may be consumed by final consumers, or else by Capitalists as intermediary products in the production of some other commodity. Once a commodity is consumed, its Use Value, is transferred to whatever has consumed it. A worker that consumes a commodity does so in order to reproduce their own labour-power. A productive process does so in order to create some new commodity. In both cases too, the Exchange Value is transferred.
The Value of Labour Power as with any other commodity is determined by the current cost of its replacement i.e. with the costs of all those commodities workers have to buy in order to perpetuate their class. The Exchange Value of a Commodity such as raw material, or a machine is preserved in the commodity it enters into. The only commodity for which this is not true is Labour-Power. Labour-Power is paid for at its Exchange Value, but the worker is able to work for longer than is required to create sufficient new Exchange Value to cover that payment. In other words, it is able to create a Surplus Value.