If something is free then what is the product?

The question is rephrasing of ‘if something is free, you are the product’ statement. And the answer can be extracting from the following:

“The existence of a free good signals that there is a companion good [across the multi-sided market], that firms consider both products simultaneously in maximizing profit, and that commonly used methods of [analysis] probably need to be adjusted to properly analyze two inextricably linked products. (Evans, 2011)”

Source: OECD (2015), Data-Driven Innovation: Big Data for Growth and Well-Being, OECD Publishing, Paris.