Product code auto generation

The product codes are already generated automatically when the product is created.

If you want to do something when the order status changes, the best is to create a hikashop plugin and implement the onBeforeOrderUpdate or onAfterOrderUpdate events as explained in the developer documentation:
www.hikashop.com/en/support/documentatio...r-documentation.html

You can also edit the save function of the file administrator/components/com_hikashop/classes/order.php directly but your modification will be erased when you update HikaShop in the future.

If you want to have other types of product codes, you should create a hikashop plugin and implement the onBeforeProductCreate event:
www.hikashop.com/en/support/documentatio...r-documentation.html
There, you can just set yourself the product code in the product_code attribute of the object given and that will use your product_code.

The plgHikashopAcymailing plugin triggers the filter system of acymailing when the status of an order is changed. With the filters of AcyMailing (with the enterprise edition) you can save a filter to subscribe someone to a specific list and set that filter to run when an order is confirmed.
Then, thanks to that plugin, the filter will be triggered automatically when orders are confirmed and your customers will be subscribed to the list you want at that point. So it seems to do exactly what you want.

The product save function triggers an event before and after saving a product. If your plugin implement these events, they will be called automatically if you plugin is published in Joomla.
There is nothing special to do as long as you do your plugin like the other plugins already available.

You can't reuse the function we use for the order number generation. The code will have to be different as the order number generation code needs the order info, and wouldn't use the product info.