is the ultimate self basically like all the dead kanayas/alternate kanayas being one with the 'main' kanaya? like let's say all those dead kanayas are all part of the one we see at the ending. thats always how i thought of it

It means that even alternate versions of character have influence over the overall arc of the character and their development, that all iterations of a character form the collective concept of that character- That ‘Kanaya’ in the grand scheme of things, conceptually, is every single Kanaya of every Timeline ever, not the Kanaya at the end specifically, for example.