Right, the actual database.
I would suggest the starting point if you are looking at a library screen edit activity is in calibre/gui2/library/models.py. Specifically the _set_data() function which I believe all the delegates end up channeling through. You can see some special cases in there for some column types, but authors fall into the general case of calling self.db.set() which I believe will go to the set() function in calibre/library/database2.py
In the case of authors then you will end up in that same database2.py file in set_authors() which calls set_path() after updating the library.
Thats my guesses from a quick look at the code without putting any print statements of my own in to verify
. I'm sure Kovid/chaley can correct me if I'm wrong which is equally possible.