Each college has its own Showcasing App with its own content and showcases, just like a brochure.
However, you can switch between the colleges by going to More > All my sites to access colleges you've visited before ***
If you want to keep showcases across devices, then go to More and Create Account. Any showcase you create whilst logged in will be visible on any device.
*** Your device's browser will remember Showcasing Apps (Sites) you've visited before and any sites you've visited will be showing in "All My Sites"