Court decisions do not decide what is unconstitutional and what is not. Something is unconstitutional or it is not and the Court confirms it.
They confirm or deny it... either way.
And they certainly do determine exactly what is Constitutional and what is not, by how they interpret what the words mean. It happens all the time.
You get two sides that disagree on what the Constitution means when it says something in very specific words, and the Supreme Court comes in there and makes a decision and sometimes it can be for one side or the other side, or even say something that neither side envisioned.
Again, it happens all the time.
That's exactly what happens, in practice (i.e., in "real life") with the Supreme Court and the Constitution.
NOW..., if the people of the United States decide, themselves, that they don't like what the Supreme Court said and they disagree with it and think that the Constitution actually meant something different than what it was interpreted to mean in some court decision that they gave -- then -- the people can propose a Constitutional Amendment, which can be worded in such a way as to give the "proper interpretation" to the Constitution, and then they essentially "override" the Supreme Court decision -- if -- the Constitutional Amendment gets passed.
That's the way it works in real life.
The Supreme Court is the final word on the legal matter in regards to the Constitution and then the people of the United States can amend the Constitution, if they don't like the Supreme Court decision (or disagree with it).
Finally we agree, basically. But there is another way, and one that has happened many times - the Supreme Court revisits a bad decision and corrects it. We need more of that if we are to ever escape from under the corrupt decisions of the Roosevelt era Court.