Sounds like the developer created a small file with one, (or a few) layouts and some scripts, then used this file as the starting point for each new solution, duplicating layouts and scripts from those already created before editing them into an actual part of the solution.
Not as much need for default layouts anymore as you can accomplish some of the same by creating or selecting a theme to use for your layouts.
File > Save a copy as > Clone (no records)
You can clone any file you want to make a copy of it as a "starting point" for new projects. A lot of companies use this, especially since the advent of custom templates.
One good starting point I have used in the past is the filemaker standards template:
Also, like philmodjunk alluded to, as long as you have a standalone .fmp12 file (NOT currently hosted by filemaker server), you can just copy and paste it in finder / explorer as you would any other file for a duplicate copy. The cloning method removes any records from the file though, leaving you with empty tables.