The Constitutional text is very broad:
The President … shall have Power to grant Reprieves and Pardons for Offences against the United States, except in Cases of Impeachment.
So it looks rather absolute, for Fedral crimes. However, the real situation is complicated. This is just one clause in the Constitution, while the President is supposed to be bound by all of it. So, presumably, he can’t exercise his pardon power in a way that violates something else in the Constitution. If you go deeper into the Federalist papers, it’s quite clear that the Founders held that no man should be his own judge, and a self-pardon effectively does just that.
Here is a good write-up, although I do note it was written before the Supreme Court put their thumb on the scale and said he could do whatever the hell he wanted, as long as he doesn’t get impeached for it:
https://protectdemocracy.org/work/the-presidential-pardon-power-explained/
I expect him to do it anyway. It will be challenged, but courts will reject it due to “lack of standing” and sidestep the messy business of having to tell the King he went too far.
The American Constitution says that Presidents can’t accept gifts from any foreign source, and that has been interpreted in the past as a general prohibition on Presidents operating in any capacity in any private enterprise. Jimmy Carter put his peanut farm in a blind trust.
Not only was Donald Trump allowed to circumvent this during his first term, retaining ownership of his businesses and nominally putting his kids in charge while they pursued foreign deals, but today Trump is waist deep in Crypto, and owns a majority share of a publicly traded company whose ticker is his initials. Foreigners can (and likely do) shovel money into both. Do you think anyone will ask him to divest, like the Constitution requires him to?
The Constitution is useless unless it is enforced. It relies on checks and balances between competing branches, and right now they are broken. The only checks on Presidential power are the military (whose oath is to yhe Constitution, not to any one President) and the individual states (who retain all powers not explicitly given to the Federal government).