Question

Is there a shared pool of memory or a shared directory in iOS where applications can read and write files?

Answer 1

No, each application is sandboxed. Applications can only read and write files/data within their own sandbox.

From the File System Programming Guide: File System Basics:

Every Application Is an Island – An iOS application’s interactions with
the file system are limited mostly to the directories inside the
application’s sandbox. During installation of a new application, the
installer code creates a home directory for the application, places
the application in that directory, and creates several other key
directories. These directories constitute the application’s primary
view of the file system. Figure 1-1 shows a representation of the
sandbox for an application.

If you need to open files of a particular type in your application, you can register your application with the system letting it know what types of files you support. Files are shared with your application through an API, not a globally shared directory. Take a look at the Document Interaction Controllers for more information on how to do that.