Creating a new product page template

1. From your Shopify admin, go to Online Store > Themes.
   
2. Find the theme that you want to edit, and then click Customize.
   
3. Open the drop-down menu at the top of the page.
   
4. Use the dropdown menu to select a template. Click + Create template.
   
5. Give your template a unique name.
   
6. Using the dropdown menu, select which existing template you want to base your new template on.
   
7. Click Create template.

See Shopify Documentation for more details.
https://help.shopify.com/en/manual/online-store/themes/theme-structure/templates#create-a-new-template