The president is the ultimate authority, but he relies on qualified experts in the affected field.
Policy is that technology is born secret, but it is not an ironclad rule.
Truman and Eisenhower sidestepped that often. Clinton side stepped it routinely for China.
There are specific laws that govern how classified information is to be handled and the president and Congress are subject to those laws (everyone but Hillary). I think what President Trump is currently doing is ordering his bureaucrat subordinates to turn over documents to the WH that they have improperly refused to provide to Congress.
He is not making some sweeping declassification of the documents. A WH spokesman said this would be a process, and the documents would be closely reviewed to determine if any part should remain classified. They will review the documents to see what the bureaucrats have held back improperly to cover their own assess and their bureaucracies rather than for legitimate national security reasons.
But the notion some seem to have that a president can declassify anything is nonsense. And some in the media are practically saying a president can declassify anything they choose.