how to make it little more clean

the last parameter in $this->load->view() is used to not display the view but return in see this

$data['main_menu'] = $this->load->view('layout/menu', null, true);
// here you get the HTML for the menu that is in app/views/layout/menu.php

$data['main_content'] = $this->load->view($content_name, null, true);
// here you get the HTML for the currently displayed page

and this if you don't know <?=$main_content; ?> is <?php echo $main_content; ?> see this

and app/views/layout/view.php is the template you can make anything in it

also this is an example you can make it in many other ways

PS: i'm not really good at explaining