The opening of the preamble ("We the people of the United States ... ") suggests that the power of this government is derived

This line in the Preamble indicates that the power of the government should come from the people of the country, or in other words, from its citizens. It states that the people are the ones who are writing the Preamble, as well as granting the government its rights and responsibilities. This idea of popular sovereignty was a common idea derived from the philosophy of the Age of Enlightenment.

By the people because the people who started the U. S. wanted the government to give the power to the people.