Why does the action hook work, which is not in the documentation?

Question

WordPress

Hello! I'm getting into Wordpress and writing simple plugins... At the moment I'm smoking the topic of "hooks"... The
question arose:
1) we have filter the_title (namely, filter, this is important... dock
from the documentation:

apply_filters( 'the_title', string $title, int $id )

2) there is a simple plugin

// код плагина
add_action('the_title', 'my_function');
function my_function($title){

  $mytitle = $title.'-суффикс';
  return $mytitle;
}
//do_action('the_title');

Now, attention to the question:
1) why does it work through add_action (and not through apply_filters, as stated in the doc)
2) why does it work without calling do_action?
I want to understand all this magic...

Answer 1

Because add_action is the same as add_filter with one peculiarity - the first one does not return anything, and the filter must return the first argument