I am native Californian so I know nothing about the contraction “y’all” except that I like it. I have been told by more than one person that “y’all” is singular and “all y’all” is plural. That defies my logic, but logic has nothing to do with dialect that becomes adopted.
I don’t know the truth, but due to the disparity between what you are telling me and what others have told me, perhaps “y’all” is used as plural by some and as singular by others. I have most certainly been told it is singular and plural is “all y’all”. Take that for what it is, because I would not at all be surprised if various people or areas of people from the south of USA use it differently, some singular, some plural.
I understand clearly that “you all” or “all of you” is plural. It can’t possibly singular. I know y’all is a contraction of you all. But I have been told by some southerners that y’all is used in the singular, so apparantly at least some people use it that way.
Spoken directly to an individual, it's almost like the British concept of a corporation being a plural entity... "British Petroleum are" or "Ford are." The inference is you, and yours.
Spoken in the form of all y'all to a group of unrelated people, it's a recognition of multiple, plural entities. You and yours, plus you and yours, and you and yours, and so forth.
You are subconsciously being regarded as something of a representative of more than just yourself. It's almost always family related, but can also mean your coworkers, your friends, those who share your political beliefs or any other form of recognizeable bond. You and your affinity group would be all y'all. You have something in common, and are being addressed in common.
I realize this is a sort of high-falutin' description of a very humble, dialectical term, y'all, but does this make it make more sense, lol?