Who can & who can't declassify documents ?
Are presidents allowed to ?
Can vice presidents ? Can they have these documents scattered around various places ?
You are only asking this now? One would have thought you already knew the answers. That you did not say a lot about where your thinking is coming from.
A couple of things for you to consider; no one, not even a former President can declassify documents "just by thinking about it." There are different processes and rules for declassifying documents based on the content of the information in the document. Not all classified documents are equal. In all cases there is a formal process for declassifying government documents. “Declassification cannot occur unless designated officials follow specified procedures,” according to law.
There were literally tons of boxes of classified government documents scattered around Mar a Lago. Even if, as President he had the authority to declassify documents there is no chance that he declassified all these prior vacating the White House. Trump’s term of office ended at noon on January 20, 2021. Just as he could not declassify documents via his thoughts, he could not have done it (constitutionally) retroactively after moving to Mar a Lago.
There is a problem with the logistics to support many of his excuses/defenses regarding his authority to declassify documents.
And before you go there, these rules apply to Biden and Obama and any other President or former President throughout history.