Well, my advice is that you migth want to read first the documentation about development
https://openacs.org/doc/openacs-4-6-2/dev-guide.htmlFor the datamodel diagram will be really difficult to see it (I think) in that form, and you will find that you don't need to know about how everything works and what is the structure to develop.
Probably you'll want to follow how the packages are built as a reference.
And if you need to know something about anything, just look for it at packages directory:
find /path/name -name "*-oracle.xql" -exec grep whatever_you_look_for {} \; -print
(thanks to markd)
or search the /api-doc
of your installation.