[null and void]

The immediate relevance is that if he can be demonstrated to have never been a lawful president, that instantly legally invalidates his every action.

If he was a lawful president each action has to be painstakingly undone one at a time.

[Sherman Logan]

I do believe there is a legal principle that means no such thing happens, but not being a lawyer I couldn’t tell you what it is.

Except that undoing all of his acts for six years is quite literally impossible.