You need to study the VCL (Visual Component Library). This is actually divided into a sal (system abstraction layer) which is the bit we can port so we can use it on different platforms (Windows, GTK+, KDE, OS X). Then there is the "stable" (I say that advisedly) bit, which is based on the OutputDevice class, which spawns the Printer, Windows and VirtualDevice classes.

The best thing that could happen at the moment is for OutputDevice to be gotten rid entirely - it's a leaky abstraction.