Tf you are viewing this over a telephone modem internet connection, playing each voice recording involves a loading delay of several minutes. The U.S. economy is basically a market system. Buying and selling is by voluntary choice, The market system dominates the U.S. economy. But no economy, not even the U.S., is a pure market economy. The GDP does not attempt to assign a value to these non-money transactions.
Money flows from buyer to seller.
These flows happen over time. Economic activity is going on all the time, with so much stuff changing hands per day or per year. An economy with private ownership of resources has a circular flow. It might better be called a loop, rather than circular. Here's the first stage of a circular flow diagram.