The delta is 'ar' archive (see 'man ar').
The delta contains 'info', some data members (named by numbers), a script named 'patch.sh.xxx',
and optional gpg signatures.
The script recreates the new deb. See do_delta_() in the python code for more details.
a delta first 'ar' member is always named 'info', and is a text file containing some keywords and informations regarding the delta itself. [TODO add details]
TODO WRITEME. You may look into /usr/share/debdelta/debpatch.sh to understand the basics.