It doesn't expose its internals, the equivalent to UNIX file descriptors in X is the XID (Window, Pixmap, etc). Much like you can give any process any file descriptor to work with, you can also give any process any XID to work with. And like you can combine the text output of processes by redirecting their stdio, you can combine the graphical output by combining their elements (it isn't exactly equivalent of course, partly due to increased complexity inherent in GUI applications but also due to little standardization and tools, but still several of elements in an X desktop - outside GNOME and KDE that implement their own functionality which ignore X - work like that).