Trying to understand the source code of the intersection function of the Mapset module

Hello.

I apologize if my questions are very basic.
I am trying to understand the source code of the intersection function of the Mapset module.
I have encountered many behaviors that I do not understand.

First I would like to understand the following behavior:

a = MapSet.new([1, 2, 3, 5])
b = MapSet.new([3, 4, 5])
c = %MapSet{map: a}

IO.inspect(c)
# #MapSet<[:__struct__, :map, :version]>

I searched in Google and in my Elixir books for that result and unfortunately I did not find any relevant information.

I can see that in #MapSet <[: __ struct__,: map,: version]> there is a :map atom key.
The intersection function makes use of that key.

If I try to change the :map key to another name I get a KeyError

d = %MapSet{other: a}
# ** (KeyError) key :other not found
#    (elixir) expanding struct: MapSet.__struct__/1
#    main.exs:34: (file)

Intersection function:

a = MapSet.new([1, 2, 3, 5])
b = MapSet.new([3, 4, 5])
defp order_by_size(map1, map2) when map_size(map1) > map_size(map2), do: {map2, map1}
defp order_by_size(map1, map2), do: {map1, map2}

# If I inspect the parameters of the intersection function 
# I get the following results:
def intersection(%MapSet{map: map1} = map_set, %MapSet{map: map2}) do
  IO.inspect(map1)
  # %{1 => [], 2 => [], 3 => [], 5 => []}
  # How is it possible that a Map was generated with the MapSet values as its keys?

  # #MapSet<[1, 2, 3, 5]>
  IO.inspect(map_set)

  # %{3 => [], 4 => [], 5 => []}
  IO.inspect(map2)

  # I understand this part of the code ｡^‿^｡
  {map1, map2} = order_by_size(map1, map2)

  # I would like to know why it is possible to interact with two different data structures:
  # #MapSet <[1, 2, 3, 5]> and % {1 => [], 2 => [], 3 => [], 5 = > []}
  # Why we surround map_set with a Map% {} struct and still return a MapSet?
  %{map_set | map: Map.take(map2, Map.keys(map1))}
end

In the end it seems that a Mapset is an undercover Map.
Thanks.