The congress are voted for by the people. If someone wants to run for Congress, they run against other people. The people choose who they want, and vote for them when the time comes.
Members of Congress and the president are chosen by the citizens.
For members of Congress, they are directly elected by citizens. The candidate with the most votes wins. These elections happen every 2 years for members of the House and every 6 years for US Senators.
As for the president, the people vote for him/her but it is ultimately decided by the electoral college. Electoral votes are somewhat proportionate to a states population, ensuring that larger states have more influence in the election.
