how to create custom template for a product type in drupal 8 commerce
by default drupal 8 uses commerce-product.html.twig. you can find it from modules/contrib/commerce/modules/product/templates/ and copy and paste it to your template folder in your theme. 

and to create separate template file for each product type you can add --product type at the end of file name. for example if you have shoes product type then you should create a file commerce-product--shoes.html.twig.

then drupal uses the new template for shoes product type only.