Hooks do not work

I just watched Mark Fail youtube video on filters https://www.youtube.com/watch?v=A9NawFvCsKE
and even after following his instruction it seems that it does not work, what am I doing wrong?

here is my code in my functions.php in my child theme.

// found this hook on https://www.premiumpress.com/docs/
// function hook_header($c){ return  apply_filters('hook_header', $c);  }	

function ba_demo( $c ) {
	
	$ba_hello_world = "<h1>Hello world!</h1>";

	$c = $ba_hello_world . $c;

	return $c;
}

add_action( 'hook_header' , 'ba_demo' );
// add_filter( 'hook_header' , 'ba_demo' );