This may be an issue where both the executive branch and Congress have authority. The executive branch administers laws passed by Congress. Only the executive branch has the power to conduct foreign policy. Arguably responding to unfair tariffs on US goods, by imposing reciprocal tarriffs is conducting foreign policy.
However only Congress has the power to levy taxes under the Constitution. A tariff is a tax. While the president administers taxes passed by Congress he cannot arbitrarily establish or impose a tax which has not been passed into law by Congress. California has asked the courts to determine if the very old federal law Trump is claiming gives him the authority to levy a new tax (ie tariffs) selectively.