How can I customize the product details page in Magento 2? Where is the .phtml file or layout for that?
1) you can find the path of the template file by using Debug mode
Login to Admin then follow below options
Store->Configuration->Advanced->Developer->Debug->Enable Template Path Hints for storefront->YES
after that clear the cache then load product detail page, you can see the template file path of each templates
2) For finding the appropriate layouts please refer official docs: http://devdocs.magento.com/guides/v2.0/frontend-dev-guide/themes/debug-theme.html#debug-theme-layout
to customise product view page in magento2 you need to create a theme, you can find all information for new theme creation here
after theme creation, you can manage product page by following path