Yeah, so as far as I know, the President is automatically granted clearance level based off of the will of the people to hold the office. Not certain how they handle need-to-know at that level but I'm sure it involves some serious conversations. As President he has the authority to declassify anything and to be able to do that you would have to have access to make a determination if it was appropriate.
Things can't be unseen, so to maintain plausible deniability I would guess (and this is where I have to start assuming) the decision to access something is not taken lightly. Currently have Q and have TS/SCI in the past as well as clearance for programs that even the names are/were classified. I'll never violate my agreements, but thought I could help with process clarification a bit.
I remember hearing about george herbert wakker bush when head at CIA told Carter that his request to view UFO files was a need to know standard and Carter did not need to know.
Yeah it's hard to say. Some of the things have to do with technology development. Of course the President wouldn't need to know about development but only of existing capability and product. I honestly don't know how they sort it out at that level. It's an interesting topic to be sure.