As we are finding out in real time, the President has the power to try to do a million things, legal or illegal, constitutional or unconstitutional, and then whichever ones don't get pushback defacto become actual powers. Throw something at the wall, if it sticks, then it's a Presidential Power. If it doesn't, there's no consequences. Just shrug and throw something else at the wall.
The President's pardon power is enumerated in the Constitution and has been litigated extensively.
https://constitution.congress.gov/browse/essay/artII-S2-C1-3...