What is the UN’s legal authority that is unequivocally agreed to by all member nations especially the U.S. ?
The United Nations does not have any authority. If it wants to take any kind of action, it relies upon its member nations to do so.
Absolutely none. It is as friedo said.
So when you sign up do you agree to abide by whatever decision the UN makes?
Are there differant kinds of “decisions” i.e. non-binding resolutions?
Saying that the UN doesn’t have any legal authority whatsoever is not really correct. Security Council Resolution based on Chapter VII of the Charter, reacting to a threat to or breach of the peace, can indeed be binding. Those reactions can include non-military (Article 41) or military (Article 42) sanctions. Whether member states abide by those resolutions is a different matter, but they are legally obliged to abide by them because they entered into this obligation when joining the United Nations. They are, for example, obliged to enforce embargos imposed by the Security Council.
Article 43 of the Charter even includes an obligation of the member states to equip the UN with troops if the Security Council requests them to do so, but the special agreements the statute is referring to have not entered into force, which is why Article 43 is currently not of importance.
A different matter are Resolutions of the General Assembly. Those are not binding and have only recommending function.