A sitting president has a constitutional grant to classify and declassify any documents, particularly ones concerning national security. Those powers are vested upon the president by the virtue of being commander-in-chief. This power exists outside of any congressional grant and the only exceptions from the rule are those imposed by separation of powers, as per Navy v. Egan.
https://www.politifact.com/factchec...resident-have-ability-declassify-anything-an/
There is an on-going EO which formalises the procedure and offers guidance, but as chief executive the sitting president can override any prior executive order, or amend existing ones. As it stands, unless the document happens to fulfil the requirements of an exception, the president can in fact declassify anything without any process and without guidance or approval. If a president says a document is declassified, 99 times out of a 100 it is the moment they said so.
That’s not in question here - the big caveat is whether Trump did declassify them or not - his claim is that he had a “standing order”, which is the part that’s being questioned. He can’t just say that a document is “declassified” because he got caught with it, that’s not how it works. While he can “declassify” documents with, quite literally, a thought, he’d have to do it *while he was a sitting president*. Since there’s no paper trail here, there’s no evidence to suggest that the documents were declassified. On the flip side, that doesn’t invalidate Trump’s claim either since, again, he is not required to consult anyone - it’s simply prudent to do so in the event that you end up with a couple of boxes of documents that, until that moment, nobody expected you to have.