i have the following liveview form

<div>
  <.form let={f} for={@changeset} phx-change="validate" phx-submit="save" phx-target={@myself}}>
    <%= label f, :username %>
    <%= text_input f, :username %>
    <%= error_tag f, :username %>
    <%= submit "Save" %>
  </.form>
</div>

the corresponding .ex file for this form is

defmodule PentoWeb.Components.SubmitForm do
  use PentoWeb, :live_component
  use Ecto.Schema
  import Ecto.Changeset

  embedded_schema do
    field(:username, :string)
  end

  def changeset(survey, attrs \\ %{}) do
    survey
    |> cast(attrs, [:username])
    |> validate_required([
      :username
    ])
  end

  def mount(socket) do
    user_details = %__MODULE__{
      username: "abcd"
    }
    change_set = changeset(user_details)

    {:ok,
     socket
     |> assign(:user_details, user_details)
     |> assign(:changeset, change_set)}
  end

  def update(assigns, socket) do
    IO.puts("update being called")
    changeset = changeset(socket.assigns.user_details)

    {:ok,
     socket
     |> assign(assigns)
     |> assign(:changeset, changeset)}
  end

  def handle_event("validate", params, socket) do
    changeset = changeset(socket.assigns.user_details, Map.get(params, "submit_form"))
    IO.inspect(changeset)

    {:noreply,
     socket
     |> assign(:changeset, changeset)}
  end
end

i am having trouble understanding how this form works, basically if someone can answer the following questions

1. in the form markup what exactly does the following for={@changeset} do ? i know it looks at a changeset, but what is it used for ?
2. what does the following achieve <%= text_input f, :username %>. i know it displays the username in the textbox but how does it get the value, if you look closely at the code i’m using user_details in the mount but i’m not specifying that this is the object the form should refer to, so how does it know which object to refer to ?