There is only one set of files containing the logic for these applications per installation, and those application files are located with the rest of the core product binary files.

While this is sometimes hard to tell (and is often driven by the product (s) you use), a good starting point for the discussion is to think of infrastructure logic versus business logic.

Once we have those, then a team could just do some UI development, create new linkages among business logic modules, and presto! Out rolls a new product or service.