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"Where does the Constitution grant them that authority?"

It doesn't.
But that is irrelevant.
We are not governed by the Constitution.
It is a piece of old paper.
We are governed by what thethe men in the branches of government think the Constitution says.
Individually, many of them may say that the Supreme Court has overstepped its bounds.
But that's all talk.
If they OBEY the Court, then by their obedience they establish that the Court has the authority to do as it does.
If your neighbor tells you to cut his grass for him, and you complain but you always do it, he is your boss because you have let him be your boss. The only way to demonstrate that he is NOT your boss is by telling him "No" when he demands that you cut his grass.

The President and the majority party in Congress grouse loud and long about the "abuses" of judicial power. If those judicial actions really are abuses, then they are not legal, and President and Congress need to ignore the abusive decisions and defy the abused authority.
But they never, ever, ever do. Not once in a century and a half.
Why?
Because the President who does will be impeached by Congress.
Or the Congress which does will be investigated by the Executive Branch and driven from power by the People.
At least that is the fear.
Which means that, even though the Constitution does not say that the Supreme Court has that power, it in fact DOES have that power.
Power is what power does.
The Supreme Court commands, and every President, Congress and State government since Lincoln has obeyed every order without defiance, not even once.
That means that the Supreme Court has precisely that power, regardless of what the Constitution says.

To give the mere piece of paper, the Constitution, authority, the other branches of government would have to defy Judicial power when in their own interpretations, such power is un-Constitutional.
But they don't.
And so the equality of covalent branches of government is like the sovereignty of the Queen of England: a nice constitutional myth.
The reality in England is that all power lies with the Prime Minister in the House of Commons.
And the reality in America is that Supreme Power reposes in the Supreme Court, and that no other branch of government has either the power or prestige to challenge it.