Unitary executive theory long predates George W Bush, and means no more & no less than that the executive power - i.e., all of it - shall be vested in a President of the United States of America.
This had nothing to do with the unitary executive. The argument that POTUS is a co-equal branch with an independent obligation to interpret & obey the Constitution goes back to the earliest days of the presidency, was most vigorously urged by Jackson.
The alternative is the courts simply finding that the law does or does not allow things. The question, a serious and longstanding one, is the limits of "who decides." Also recurs in when the executive can refuse to defend/enforce laws (eg DOMA).
As I said, there are serious arguments around all these points - the unitary executive component is the most obviously supported in the text & structure of Article II - but none of them were brand new debates that started after 2001.
In Trump's case, he has typically *not* used the executive power to invade the other branches, esp. compared to his predecessor. He's thrown around very broad visions of a unitary executive but has typically just whined about it on Twitter instead of using the power.
