A product detail page is created whenever you add a product. It contains the basic product block.
When you install the eCommerce plugin, you get an additional page type for showcasing your products called "Product Detail." It is a completely normal concrete5 page, but it has a product block included by default. When you make a new product, a new page of this type gets created and is set to feature that product.
To add content or change the look of an individual product's page, you can reach it from your sitemap on the dashboard. If you want to change how all your products will look, you can modify the page type default for "Product Detail" from "Page Types" under "Pages and Themes" on the dashboard.
Editing a page type default is the same as editing a normal page, except when you apply your edits, all new pages of that type will have the modifications you just made. To add a change to all your existing pages with that page type, click on that block and choose "set up on child pages"
The only thing you have to consider when doing this is where your product block is going to be. When you put the page in edit mode, you can see it as a red dotted bar.
Because the block is empty (it is not associated with a product right now) it just shows up as a thin red rectangle. If you click the rectangle and choose edit you can still choose any options you want, and if you apply it across the child pages all of your product detail pages will show similar data.