Originally Posted by
ringpriest
(Disclaimer: I'm quite a few years away from current on any of this, but I doubt its changed much.)
US Presidents have practically unlimited authority to grant access to classified information. For example, if Biden wanted to get Obama's opinion on something classified (say, having to do with Iran), he can just say "I'm talking to Obama about this" and that's that. Obama doesn't need to get a security clearance, but neither is the information declassified. Meaning, among other things, that Obama can't blab about it to the NYT, and you or I can't file a Freedom of Information Act request and demand "whatever Biden told Obama about Iran". This, by the way, is what saved Trump's bacon back when he was disclosing classified info to Russians in the White House; the information was still classified, but he as President could share it with them.
If a President thinks they're likely to want their predecessor's opinion, or sometimes even as a courtesy or gesture of respect, they can arrange it so that the former President gets regular briefings on classified information. These are solely at the discretion of the President, and don't directly relate to any security clearance the former President. Note that with all of this, the information in question is not declassified. It's not released to the public, and it is still a crime to mishandle it.
Now, the President of the United States also has very broad authority to order materials declassified - but he doesn't get to just wave a magic wand and say "this is declassified, and that is declassified, everything is declassified!" There are laws (passed by Congress), executive orders (issued by various Presidents), and a whole host of regulations. For most classified information, the President can say, "declassify this" and direct his various subordinates in the executive branch to make it happen according to established processes. If they refuse ("I'm refuse to declassify the location of all our spy satellites, Mister President!") then he theoretically can fire them and replace them with someone who will (as long as such replacement is done according to the law as established by Congress - there are limits on who can be Acting Secretary of Defence, for example - limits that Donald Trump was pushing hard at the end of is time in office). There are a couple exceptions to this; for example, most nuclear secrets are classified by act of Congress, and if the President tries to do an end run around that, it's something of a constitutional crisis.
All the above is about classified information. The crimes Donald Trump appears to have committed go well beyond abusing classified information. He's also violating the Presidential Records Act (which was passed after Nixon, to, among other things, make sure former presidents didn't abscond with records of their crimes...), and he's near-certainly in violation of the Espionage Act, which is, among other things, about proper handling of National Defense Information. NDI is anything that impacts the security of the United States. Classified information certainly can be NDI, and vice-versa, but even if Trump had declassified (which he almost certainly didn't, because there would be a paper trail) lists of spies, nuclear secrets, military programs or anything along those lines, they would still be NDI, and it would be a crime to do what he's done with them (a whole laundry list of crimes, really).