What exactly is t() (except type specification)

It’s a way to reference types. It might look like a function call, but everything after @type, @spec, … deals with types and types only. It’s not going become executable code.
For a more complete explanation take a look at this one: Typespecs reference — Elixir v1.20.2

Thank you LostKobrakai. I read typespecs on hexdocs before I asked. I was just thinking that there is underlying structure. For example String (if you just type it to IEX) is actually an atom, Struct is a map with extra fields…

If there is no underlying structure (like it’s not a function) it means that I can actually create a function t() or struct field “t” without braking something.

t() is basically a convention. You can just as easily have this module:

defmodule MyStruct do
  @type my_struct :: %__MODULE__{}

  defstruct [:x, :y, :z]
end

And then reference the type in foreign typespecs (= typespecs in different modules) like so:

def ModuleUsingMyStruct do
  @spec use_the_struct(MyStruct.my_struct()) :: map()
  def use_the_struct(%MyStruct{} = x) do
    Map.from_struct(x)
  end
end

You can see for yourself that this introduces some annoying extra typing so a convention emerged where t() is being used, namespaced within the containing module (example from standard lib: String.t()).

Yes, you can.

Thank you