The capitalist buys the labor power (power is effort applied over time) from the worker for a block of time to produce commodities. The worker contributes some labor power to each commodity so that the value of the commodity is partially made up of that particular worker's labor. The commodity is made of raw materials and other parts that were made or picked or mined by workers, and the machines that are used were also made by workers, and a little value of the machine transfers to the commodity too. So everything that comprises the exchange value of the commodity, is made up of worker labor power.

The capitalist doesn't want to sell the commodity at a loss, so the faster the worker produces commodities for the capitalist, the more money the capitalist makes. Its also always better for the capitalist to get more work out of a fewer number of workers. The capitalist doesn't want to break even either, so the capitalist has the worker produce more commodities than what it costs to buy the workers labor power for that period of time. So for some of the work day, the worker is making commodities that will pay for their own sustenance wages, but for the rest of the day the worker is making commodities for the capitalist, basically the worker is working for free for a large part of the day.

The value created by selling these commodities made with free labor is reinvested in more capital that makes more money for the capitalist. This is why everything that the capitalist owns, all of their capital that they use to make profits, is paid for with the labor power of the workers.

https://anticapital0.wordpress.com/calculating-surplus-value-to-facilitate-workplace-organizing/