Where are the default emails? I am using Drupal Commerce in Drupal 8. 

Where can I find the default email and rules related to order activities, like on completion, processing etc.
Email templates can be found in module subdirectories:
https://docs.drupalcommerce.org/commerce2/developer-guide/orders/customize-order-receipt-emails

For D8,  commerce is not using rules any more, but states machine and event subscribers, check order workflows / checkout flows 
https://docs.drupalcommerce.org/commerce2/developer-guide/orders/workflows