In many instances the words are the same.
Private (vs public) comes from early Latin and is meant to distinguish between matters that are of public concern vs those that are handled or controlled privately.
The current meaning of privacy may also mean information that is necessarily about oneself that the person feels a need to limit knowledge of. This is like a secret but a secret may also be some information about anything, not necessarily information about a person, like a trade secret. You wouldn't normally say that a trade secret has to do with privacy, you say that it is secret.
Also, privacy is like a natural instinct, whereas secrecy is a reasoned construct with a conscious purpose behind the reason for the secrecy. Privacy is a thing that has evolved with our species (and many other species) through natural selection over time.
Thank you.
Privacy is one of the reasons behind Secret.
Trade Secret is also a reason.
For not public and hidden, I think that
in some instances,
not public means others cannot access or know the content of it,
and hidden means others cannot know the existence of it.
@blinkwarp
Thank you for you explanation.
Isn't secrecy still noun?
So the problem is converted into what the difference between not public and hidden is.