The apps are launched on the compute nodes and accessed through the web interface. To launch an application select an apps from the Apps menu. In the app form you will be able to select billing project, partition, resources and app specific settings. After launching the application you will see the My Interactive Sessions page shown in the screenshot below.
On that page you will be able to connect to the app. Note that apps keep running and consuming resources even if you close the tab for the app session. To stop the app you can delete the session in the interactive sessions page.