how does a html form work exactly?

[mp3909]

I have been trying to teach myself Python Django.
Been following videos on YouTube.

I'm confused in understanding how this piece of code that has been added to create a form, works exactly.
Before I explain, I would like to point out that my model for the Song class has a Boolean field called is_favourite with the default set to False.

Here is the code for the form:

<form action="{% url 'music:favourite' album.id %}" method="post">
  {% csrf_token %}
  {% for song in album.song_set.all %}
      <input type="radio" id="song{{ forloop.counter }}" name="song" value="{{ song.id }}">
      <label for="song{{ forloop.counter }}">
          {{ song.song_title }}
          {% if song.is_favourite %}
              <img src="{% static 'music/images/star.jpg' %}" width="20" height="20">
          {% endif %}
      </label><br>
  {% endfor %}
  <input type="submit" value="Favourite">
</form>

I have attached an image of what the form looks like when I run the development server (it is boxed in red).
The purpose of the form is to allow users to "favourite" their songs.
When they have favourited their song(s), they click on the "Favourite" button which I believe submits all of the form data to the url 'music:favourite' which calls views.favourite function:

def favourite(request, album_id):
    album = get_object_or_404(Album, pk=album_id)
    selected_song = album.song_set.get(pk=int(request.POST['song']))
    if selected_song.is_favourite:
        selected_song.is_favourite = False
    else:
        selected_song.is_favourite = True
    selected_song.save()
    return render(request, 'music/detail.html', {'album':album})

I understand that the code in the form runs before the associated function in the view is called. Having said that, it is this line in the form code that I can't get my head around in understanding how it works:

{% if song.is_favourite %}
    <img src="{% static 'music/images/star.jpg' %}" width="20" height="20">
{% endif %}

I understand this is saying if the field "is_favourite" of the Song class is set to True, then add a little star image next to that song. But all songs are set to False when you try to run the form for the first time. So that line of code does not get executed i.e. a star image is not added to any of the songs. But let's say a user selects a song as a favourite and then clicks the submit button, does it only send that particular song the to the function in the view? If so, how does it know to only send that song, is this something to do with the mechanics of how "radio" button in html forms operates or am I missing something? Aside from this question, if the function then sets the field value of "is_favourite" of that particular song to True, then how does the "star" icon get added in the first place? We have already executed the lines of code in the form code and then we went to the view function which set the property to True so unless a call is made back to the code in the form to set the star icon, I don't quite understand.

Thank You in advance.