The answer requires some grounding of the nature of treaties and international relations (IR). I've found that in general that people are swimming in seas of thought without know that they are doing so and what the nature of that particular sea is, how it got there, whether it has changed over time, and if there other seas. Subsequently they often try to puzzle things out from first principles without knowing
Just as a proof is a proof, a treaty is a treaty and an MOU is a MOU. A MOU can become a treaty but that's only because a nation has decided that it's a treaty and obliged itself to it.
There's a caveat to that, which will take up the next 3 paragraphs, in that the IR community has agreed that there are some universal laws, which all nations and all persons are obliged to keep, but it's interesting that the people most keen on using them to try to bell certain cats are those who are rather selective about what universal rights exist – the Lockean triad of property rights, the liberty of the subject, and basically being able to exist being particularly nettlesome for them-- and are substantially rather more keen on the concept of positive law rather than natural law to the extent of trying to suppress the founts of the idea of natural law to the extent that although saying 2+2=4 is not yet a problem, stating that a man is biologically a man and a woman is biologically a woman is.
It must be pointed out that the concept of a universal arises out of Western political thought, which is comprised of two major sources: ancient philosophy and Christianity, often metaphorically referred to as Athens and Jerusalem. It doesn't matter if you like that reality or not; it is reality, just as real as the existence of meteor impact craters on the Moon (and the Earth for that matter).
The UN declaration of universal Human rights, at the high water mark of the confidence and influence of the Western world –that is to say Western thought - after the second world war but before the corrosion of things like relativism and the Frankfurt school etc., arises out of that.
Apart from that, the society of nations in the Westphalian world is contractual in nature; a nation enters into an agreement with one or many other nations to do {something} and they, as a consequence, enter into obligations (and benefits ) from whatever the treaty is about.
For example, no matter how much the citizens or government of country A and country B may not like it, there is no obligation for either country to provide relief from double taxation and to map between entirely different taxation schemes.
An example of that is that Americans resident in Canada very soon learn that the US tax law doesn't know about things like RRSPs and TFSAs, doesn't want to know about them, put on its noise control headphones, shut the door of its office, and is currently drafting a email to HR complaining about a hostile work environment.
Similarly, just as a private contract between persons A and B cannot impose a duty on person C (although it can grant benefits), a treaty cannot oblige a country which is not a signatory to the treaty.
When does a treaty exist and come into effect and when does a nation become obliged to it?
It entirely depends on the internal laws of a nation.
A nation only enters into a binding treaty when it has performed the actions which law and custom of that nation require of it.
For example, in theory Her Majesty the Queen in right of Canada can enter the Dominion of Canada into a binding treaty but the constitutional practice is that it requires the consent of Parliament.
http://publications.gc.ca/Collection-R/LoPBdP/BP/prb0004-e.htm
https://treaty-accord.gc.ca/procedures.aspx?lang=eng
Similarly, the United States Constitution has explicit requirements about how to enter into a treaty. No matter how popular a particular president is, how many celebrities, news people, and other bien-pensants are in favour of something, and how many nations have signed an agreement, if the agreement has not been through that process then it isn't a treaty under United States law and no amount of snickering, mockery, and contempt alters that.
“A proof is a proof. What kind of a proof? It's a proof. A proof is a proof. And when you have a good proof, it's because it's proven.”
Read more at: https://www.brainyquote.com/quotes/jean_chretien_145285
