SFX is the "old" framework, used for historical reasons.
An attempt of documentation of this module is located in [git:sfx2/doc].
It contains base classes for document model, view and controller, used
by "old" applications like sw, sc, sd (while "new" applications
are based on the "new" UNO based framework in "framework").
The SFX framework is based on dispatching slots identified by integers
(SlotIDs) to SfxShells, and there is a dedicated IDL compiler (svidl)
involved that generates C++ slot headers from SDI files in modules' sdi/
subdirectory.
Documentation about SFX dispatch, SDI etc.: https://wiki.openoffice.org/wiki/Framework/Article/Implementation_of_the_Dispatch_API_In_SFX2
Document load/save code is maintained in [git:sfx2/source/doc/docfile.cxx]
SfxMedium class, which handles all the twisty load and save corner cases.
[git:sfx2/source/appl/sfxhelp.cxx] Start procedure for the online
help viewer top level window; handling of help URL creation and
dispatch.
There are also some UNO services here that could really be implemented
anywhere, e.g. the DocumentProperties or DocumentMetadataAccess.
Notable files:
sfx2/source/dialog/backingwindow.cxx Startcenter buttons and the corresponding event handler.