Definition of "all"
all
/ɔːl/
pronoun
(chiefly Southern US, Midland US, Scotland, Northern Ireland, India) Used after who, what, where, how and similar words, either without changing their meaning, or indicating that one expects that they cover more than one element, e.g. that "who all attended" is more than one person. (Some dialects only allow this to follow some words and not others.)
conjunction
(obsolete) Although.