Using Enum.reduce ultimately delegates to the protocol function Enumerable.reduce, which is implemented for Map here:

https://github.com/elixir-lang/elixir/blob/79388035f5391f0a283a48fba792ae3b4f4b5f21/lib/elixir/lib/enum.ex#L3351-L3353

This first converts the Map to a list of {key, value} tuples, then reduces over it.

Tangentially related, I love the StackOverflow Axiom at play here: “It’s easier to get a good answer by proposing an incorrect implementation than by asking a well-stated question.”

That won’t work, there is no += in elixir. I think what you really mean is more like this:

alphabet_positions = %{"a" => 1, "b" => 2, "c" => 3, …}
Enum.reduce(alphabet_positions, 0, fn {letter, position}, sum ->
  if letter in ~w[a e i o u], do: sum + position, else: sum
end)