i dont understand how phoenix presence works

in the examples

defmodule MyAppWeb.MyChannel do
  use MyAppWeb, :channel
  alias MyAppWeb.Presence

  def join("some:topic", _params, socket) do
    send(self(), :after_join)
    {:ok, assign(socket, :user_id, ...)}
  end

  def handle_info(:after_join, socket) do
    {:ok, _} = Presence.track(socket, socket.assigns.user_id, %{
      online_at: inspect(System.system_time(:second))
    })

    push(socket, "presence_state", Presence.list(socket))
    {:noreply, socket}
  end
end

it pushes to presence_state on :after_join event…
after that how does it work? how state is being pushed down to client in realtime as changes are happening?

is there any good way to hook into presence_state to identify which clients are online realtime?(I want to implement a total user counter)

PS: Im following this for my usecase https://medium.com/@alvinlindstam/phoenix-presence-for-social-networks-5fb67143f0ad