When you open the Pages Admin you can see all active Pages in your domain. From the left hand side you can filter Pages (not search), select which kind of Pages you want to see (restricted, domain shared, not shared), and you can select to view archived Pages.
Every page has a sharing status of either "restricted", "domain shared" or "not shared". The restricted means that it is shared to specific users, but not to the whole domain. Domain shared means that the page group is shared with the whole domain (with read rights). "Not shared" means that the page group is only accessible by the owner, i.e. it is not shared.
Page can be a primary or a secondary page. This can be changed from the page row right hand side menu. Domain can have just one primary page and multiple secondary pages.
Admins can enter the Page edit mode for any Page and change it in anyway. Admins can modify the Page content, styles, sharing settings etc.. You need to enter the edit mode from the Admin panel. This will also automatically turn on the Admin mode.