A.Enum.group_by

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

group_by(enumerable, key_fun, value_fun \\ fn x -> x end)

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Specs

group_by(t(val), (val -> key), (val -> mapped_val)) :: %{
  optional(key) => [mapped_val]
}
when val: value(), key: any(), mapped_val: any()

Splits the enumerable into groups based on key_fun.

Mirrors Enum.group_by/3 with higher performance for Aja structures.