Can I see source code of existing game systems?
 

I see how I can use the editor to create modifications, but to know the names of the things I want to modify it would really help to see how they are implemented.
Even if I buy the authoring kit, I see no indication that I could look into .pkg files.
Am I missing a trick? Or is what I want impossible because of licensing?

You can see the source code of Savage Worlds.
You can see the source code of user-created systems like Alternity and Doctor Who: Adventures in Time and Space.
You can see the source code of user-created supplements like the community packs for Pathfinder and D&D 5E

As far as core content with a compiled system, like Pathfinder, you have a couple of different options.

Within the editor, you can copy certain elements to see how they're coded. You can use the "Copy" button to search for things within the editor, or you can use the debugging tools to find the names of things within an active portfolio.

You can get the source code to Savage Worlds, but all other game systems are hidden inside .pkg files for copyright purposes.

Unless they changed it, 4e was also available. You can open them in a text editor. I believe the Savage Worlds files are in a folder named source, but the other game systems the source code files were not put in such a folder.