Twitter hides Donald Trump tweet for “glorifying violence”

On the contrary, the way the US constitution works is the government has no power at all, except granted by the constitution or laws passed by congress that are constitutional. The question is - which law empowers him to do so?

You are both mistaken. Laws are written by people, and the constitution is a piece of paper. The legislative body that passed these laws just heard arguments from the House that the President attempted to extort a bribe from a foreign allied government using taxpayer dollars, and on the other side they heard the President argue that not only is he within his rights to refuse handing over any and all evidence of that crime, it's not even a crime because if he believes it will benefit his election, and he believes his election will help the American people, ispo facto he cannot be removed from office.

The Senate agreed with the President and decided not to remove him for this conduct. Not only that, he cannot be indicted for said conduct because the Justice Department has a policy stating they cannot indict the President. For instance in the Mueller investigation they documented no fewer than 10 instances of obstruction of justice by the President, a federal crime. The lead investigator in that case also testified under oath that the President was not truthful in his answers to investigator questions.

This has emboldened the President to argue in court that not only is he immune from indictment, he's immune from any and all investigation.

So the question is not which laws constrain the President, nor is it which laws empower the President. At this point after all that, the question is: "Who is going to do this for the President, and who is going to stop him?"