The normal means to script Pages is with its AppleScript scripting dictionary accessible via Apple's Script Editor.
The Pages templates that Apple distributes are within the application bundle. Those that you have created are in:
/Users/username/Library/Containers/com.apple.iWork.Pages/Data/Library/Application\ Support/User\ Templates
The AppleScript scripting dictionary for Pages will give you programmatic access to the internal Pages application templates as per this example:
set newdoc to (make new document with properties {name:"foo", template:template "Blank"})
There is no guarantee that any hardcoding into the application bundle to access its templates will not break in the future should Apple name or locate them differently than present.