Kim, in the United States, employment is generally considered to be “at will”, meaning an employer can fire you “at will”, and without cause. There are a few exceptions. Here are some details; https://www.rocketlawyer.com/article/what-states-are-at-will-employment-states-ps.rl
In the US government, executive branch agency heads and the president’s cabinet serve at the pleasure of the president, and most are replaced after every new president is elected. One example I used was Clinton, who fired 93 United States Attorneys soon after assuming office.
The president controls the executive branch of government, congress controls all legislation, passes all laws, and controls government funding. The Supreme Court is responsible for ensuring laws passed by congress pass constitutional muster.
The constitution was drafted as a limiting document, identifying rights citizens possess which precede the formation of government and are specifically outside government control. It identifies specific limited rights which citizens grant government.