Well it isn’t about just a .php file. Not sure what you want to do, but here is where you should look:
in /app/design/frontend/default/default/template/catalog/product you will find the file building the pages displaying the products (mostly in view.phtml for the product and in list.phtml for the product list).
any function called in those template file refers to functions defined here /app/code/core/Mage/Catalog/
in particular you should find the functions called in view.phtml and list.phtml here :/magento/www/app/code/core/Mage/Catalog/Block/Product