When you select an inline citation, you will get a popup listing the included reference(s). Papers will recognize any articles or citations you already have in your library and mark it with the "In Library" field.
Clicking on an article title will take you to that article's webpage. You can easily add those references to your library by clicking on the "Add to Library" button, or you can view the PDFs. The "Get PDF" option will try to locate and access the PDF, if access is available.