xref_calls.exs

# My GenServer

defmodule My.GenServer do
  def start_link() do
    GenServer.start_link(__MODULE__, :ok, name: __MODULE__)
  end

  def init(_) do
    {:ok, []}
  end
end

# My application.ex

defmodule My.Application do
  use Application
  
  def start(_, _) do
    import Supervisor.Spec
    
    children = [worker(My.GenServer, [])]
    
    opts = [strategy: :one_for_one, name: My.Supervisor]
    
    test_start(My.GenServer)

    Supervisor.start_link(children, opts)
  end
  
  def test_start(module) do
    module.start_link()
  end
end

# Code I ran to inspect the xref graph

    Mix.Tasks.Xref.calls()
    |> Enum.filter(fn
      %{callee: {_, :start_link, _}} -> true
      _ -> false
    end)

# Outputs

[
  %{
    callee: {Supervisor, :start_link, 2},
    caller_module: My.Application,
    file: "lib/my/application.ex",
    line: 11
  },
  %{
    callee: {GenServer, :start_link, 3},
    caller_module: My.GenServer,
    file: "lib/my/gen_server.ex",
    line: 3
  }
]