Generation/transmission cooperatives produce electricity using a variety of sources, including coal, natural gas, nuclear power, and renewables, and sell it to distribution cooperatives.
A local example is Old Dominion Electric Cooperative, which produces electricity and sells it at wholesale rates to distribution cooperatives in Virginia, Maryland, and Delaware, including Southside Electric Cooperative. These cooperatives then distribute the electricity to their members. SEC does not produce electricity.
America has about 900 generation/transmission and distribution electric cooperatives. Operating in more rural areas, electric cooperatives serve 42 million Americans in 48 states.