In the International Emergency Economic Powers Act of 1977 the Congress gave the President the power to declare "national emergencies" based on what the "President believes" (section 1703 of the Act). Trump is claiming that Act gives him the power to force all American companies to leave their manufacturing and assembly operations, and their commerce with, China. He is right.
The 1977 law, which was meant to limit presidential power, did no such thing. The law says emergencies can be declared, and almost unlimited presidential power on foreign commerce can be exercised, based on only presidential "beliefs".
Only the weak and frankly vacuous instruction in the 1977 Act to "report to Congress when the President believes" (section 1703), to "consult with Congress", and to "At least once during each succeeding six-month period after transmitting a report" send another report, are the extent of Congressional involvement once a president uses this Act.