All y'all is a Southern expression inclusive of myriad groups of peoples.
Y'all is just plural to distinguish it from singular you.
I'm struggling here to explain it because historically English had thee and thou but that wasn't singular versus plural you. It was intimate versus formal you.
Tu in French is a close friend or similar. Vous is someone you don't know well.
The word they can be used in singular form as a gender neutral version of he or she if being gender neutral matters more than grammatical agreement on singular versus plural. It's generally less awkward than trying to use the word one which sounds stiff and formal and folks don't much use that construction.
In the South, generally speaking:
You = just one person that I'm speaking to.
Y'all = a group of folks I'm addressing who likely identify as a group, such as members of the same family or members of the football team.
All y'all = a group of groups, inclusive of many demographics that don't identify with each other, like Whites and Blacks and Redskins or students and staff at a school.
You might say "All y'all settle down." to mean everyone being rowdy in a situation where different groups are being confrontational.
You might then turn and say "Y'all behave." if it's a mixed race setting and the White folks imagine you didn't really mean them too.
And then you might single out one person in particular and say "You come here. Yes, you."
Footnote
What I'm trying to say is in French and German there's technically a singular you and a plural you but singular you is also intimate and plural you gets used to address individuals you don't know well respectfully. So plural you and singular formal you are the same.
Southerners aren't making that distinction. We're a friendly bunch.
But y'all and all y'all distinguishes a more or less unified group from a diverse group of groups.