A.Enum.join

You're seeing just the function join, go back to A.Enum module for more information.
Link to this function

join(enumerable, joiner \\ "")

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Specs

join(t(val), String.t()) :: String.t() when val: String.Chars.t()

Joins the given enumerable into a string using joiner as a separator.

Mirrors Enum.join/2 with higher performance for Aja structures.