I have recently install Magento 2.3. The
app/design/frontend/magento/ is empty. I want to modify the product detail page and catalog detail pages. I am new to Magento can anybody help me in this
You need create a new theme based on Magento luma or blank. please read through Magento Doc:
You could follow that tutorial to create your own theme (MyCompony/mytheme). And copy all files in (vendor/magento/module-catalog/view/frontend/templates/product/) into your new theme folder (app/design/MyCompony/mytheme/Mage_Catalog/templates/product/). If you want to edit category page just modify the list.phtml. For Product detail page, you have to modify the corresponding files inside product/view/.