President Trump can fire most anyone he wants in government.

No, he cannot.

Under the original Pendleton Act and the Civil Service Reform Act, nearly all government employees can only be fired for cause. Again, political junkies tend to think of policy-adjacent folks when thinking about federal workers - but the overwhelming majority of the federal civilian workforce consists of people who are doing jobs like working in a VA hospital or support roles on a military base or maintaining the grounds on federal lands or the like. They can't just be fired because the President deems them inadequately "loyal" or "supportive of the President's agenda" - in fact, they're not allowed to be fired for any reason that isn't based on merit or job performance. Not only can they not be fired at the whim of the President, but they have both statutory and constitutional rights to a formal due process review to make sure they get their jobs back if they are fired for an illegitimate reason.

The President is not a king. Nearly all of the agencies that make up the federal government were created at the direction of Congress, and Congress has passed laws that direct how those agencies are to be staffed and how people are to be fired if necessary. Because the President's job is to execute the laws, not make them, he has to follow those laws in managing the federal government. It is not like the closely held private corporate structure he is used to.