In Opencart 1.5.X email notifications for new order\customers\etc was implemented via catalog/model/checkout/order.php in confirm() method. But in OC3.X this code was completely rewrited and I can not find how this functionality is implemented in this version.
Your list of notifications implements in different places. Email notifications mostly implements in /catalog/controller/mail/
There you can find affiliate, forgotten, order, register and transaction.
Each controller related to one or more different models. You can find a related models using search in these controller, looking for $this->model_.
For example /catalog/controller/mail/order.php contains references mostly on /catalog/model/checkout/order.php ($this->model_checkout_order).