No prob! It’s a bit confusing at first, especially since it’s set up to look like submodules.
What’s even more confusing is that a nested defmodule will auto-alias and then in that case you can do what you’re talking about 
defmodule Foo do
defmodule Bar do
defstruct [:id]
end
%Bar{} # This is ok
end
Also some macros, like Phoenix.Router’s scope can cause confusion around this.
EDIT: D’oh @garrison beat me to it 
Also so long as I’m blabbing on here (I used to be quite confused about this myself) it could either help or be more confusing to understand that Foo.Bar is syntactic sugar for :"Elixir.Foo.Bar". This makes it really obvious that the . is more of a random character treated specially only by some macros. Playing around in IEx may help:
iex(1)> :"Elixir.String" == String
true
iex(2)> :"Elixir.String".capitalize("foo")
"Foo"
iex(3)> defmodule Foo, do: (def hi, do: "hi")
{:module, Foo,
<<70, 79, 82, 49, 0, 0, 5, 64, 66, 69, 65, 77, 65, 116, 85, 56, 0, 0, 0, 164,
0, 0, 0, 17, 10, 69, 108, 105, 120, 105, 114, 46, 70, 111, 111, 8, 95, 95,
105, 110, 102, 111, 95, 95, 10, 97, 116, ...>>, {:hi, 0}}
iex(4)> :"Elixir.Foo".hi()
"hi"
It’s worth pointing out that the reason for this is to ensure an Elixir module name will never collide with an Erlang module name.
It might also help to point out that alias Foo.Bar is shorthand for alias Foo.Bar, as: Bar. Both mean that within that scope you can use Bar to refer to Foo.Bar. But, you can name the alias something else like alias Foo.Bar, as: Baz and then use Baz within that scope