"Everybody prospers" in a free market. Why?
Because the results of exchanging
goods and services are the ones each person chooses. "Prosperity" is about fulfilling wishes, getting choices satisfied.
Because each participant gets to keep all the products of his own labor or payment, he has the motive to make himself more and more productive
Competition adds an extra incentive
to improvement, and so provides buyers with more for their money; ie, more prosperity