This should be a simple question. On websites like themeforest where you can buy themes others have built, when you preview the themes almost all the themes will have seperate pages for you to view seperate functionality
For example if a theme has a portfolio, a blog with sidebars, a blog without sidebar, a blog with 2 sidebars. They will link you to different pages to see what each looks like.
How do you do this? I mean if I just build a file called portfolio.php and put all my code to build that page, if I go to portfolio.php in the URI it isn't going to load that page, so how can I access a separate file, like portfolio.php