Sorry .. The answer depends on each enterprise set up.
I found the library files in our $AU_TOP path
and I kept inside my forms 11g .
Yet to test this .. but this is the solution.
You cannot develop in forms 11g for 12.2.5. You need to install 10.1.2.0 then patch your developer client to 10.1.2.3. Then you can upgrade developer 6i forms to 10g forms and successfully compile in 10g on 12.2.5.
Oracle E-Business Suite Release 12.2: Consolidated List of Patches and Technology Bug Fixes (Doc ID 1594274.1)