This Texan's understanding:

"Y'all" = plural you

"All y'all" = "all of [plural] you" as distinct from "some of you/y'all"

Don't think I've ever heard a singular y'all. "How ya doin'" would be the singular version of "How y'all doin"

For me, "y'all" has multiple forms

"y'all" - explicitly addressing >1 person

"y'all" - explicitly addressing 1 person while implicitly addressing >1 people. I might say "how y'all doin" to a friend, implying "how are you and your family doing"

"all y'all" - >>1 person or addressing >1 person that was not included in the previous "y'all"

"y'all" - explicitly and implicitly addressing 1 person whom I have not made acquaintance with. If I were working as a server in a restaurant and had a single person come in, I may address them with "y'all". "What can I get y'all?" is the same as "what can I get you?" but the "y'all" gives it some extra politeness or an "emphatic southern accent"

This often comes up in such discussions, but no, both "y'all" and "all y'all" are plural. They're useful for distinguishing between smaller and larger groups. For instance, if a couple of members of a family are visiting, you could say "you" (the person you're speaking to), "y'all" (the whole group visiting), or "all y'all" (you and the rest of your family who aren't here). Or if multiple groups are visiting, "y'all" might be one group and "all y'all" might be everyone.