There are may ways to restrict the users access to these pages that I will explain later in a different post.
Below Im listing down pages ; those are available in each site unter layout mapping.
Site Collection and Site Level Recycle Bin Urls/_layouts/15/AdminRecycleBin.aspx
User , User Info , Site Permision related Urls_layouts/15/permsetup.aspx
This is used to create default groups in SharePoint such as visitors , members and owners.
All People Url , This can be even used to delete users from the site collection
User Information Hidden List Urls/_catalogs/users/detail.aspx
These lists are actually available from previous version of sharepoints. These are hidden lists avaialble in the sharepoint containing user infomation.
This is used to view user information in the same page without redirecting to my site.
This is used to manage quick launch items in sharepoint.
This is used to modify navigation setting in the site colletion.
Workflow history hidden list
This is hidden list for manage and view workflow history items.
Site usage page/_layouts/15/usage.aspx
This is used to view the usage information. but nothing harmfull.
Site content and structure pages/_layouts/15/sitemanager.aspx
This will bring you site management page which is available from sharepoint 2007.
List of heirachy pages
This is the site setting page that contains all administrative links.
This is used to view all site contents. List’s visibility if this page can be hidden using shareopint designer.
Site and Site Collection feature management links
These links are used activate and deavtivate features in site and site collection.
SharePoint designer Settings
This is used to enable and dissable sharepoint designer settings.
Look and Feel Urls/_layouts/ChangeSiteMasterPage.aspx
This is used to change site collection master pages.
This is where user can change Page Layouts ,Site Templates and other related settings.
This is main document library for update and maintain related files and master pages