The president has top secret clearance and doesn't have access to personal tax returns. I don't believe there is a blanket "all things included" security clearance.
This is correct. There is no "you get access to everything" kind of clearance. That includes the President. Congress has the power to get access to anything, but even then there is a tight process limited to a handful of congresspeople.
A core part of this sort of security is compartmentalization. A person who has "top secret" level clearance has it for specific subject matter. Their clearance is worthless if they're outside those boundaries.
"The President of the United States will be given access to any government or military information that they request, even if they would not otherwise be able to normally obtain a security clearance were they not the President."
Trump essentially granted Musk his presidential clearance. If Musk hits any resistance to classified information, Trump can just access it and hand it to Musk. If it's a crime to do so, Trump can declassify it, pardon Musk, or just instruct his DOJ not to charge him.