Technically, he does have the authority which is derived from several legislative acts, like the Trade Expansion Act of 1962 and the International Emergency Economic Powers Act. They allow POTUS to adjust tariffs for national security reasons or during national emergencies.
There’s also precedent for this authority being exercised, as recently as 2018 when Trump imposed tariffs on steel and aluminum imports from various countries, including Canada and Mexico, citing national security concerns. These led to retaliatory measures from the affected countries and were a significant factor in the renegotiation of NAFTA, resulting in the US-Mexico-Canada Agreement (USMCA) in 2020
thanks for the detailed response, appreciated!