Just because the President has certain powers, doesn't mean he will be allowed to abuse those powers and remain unchecked.
If Trump tried to force all American companies to stop doing business in China by declaring China a "national security threat," his efforts would be immediately met with a challenge from the courts and he would very likely be impeached or removed from office. What would not happen is the rest of the government simply throwing up their hands because "the president controls the defense apparatus."
If it really worked that way, Trump would already have declared Mexico a national security threat, forced the government to fully fund the wall, seized all necessary lands via eminent domain and started building it by now.
It works so well that Trump is maintaining concentration camps where children are physically abused. And his appointments in the supreme court are letting him bypass congress and build a useless wall with money diverted from the military. If you think that this is normal, then you're part of the problem.
