There is a file that is a databse of every field in every file
in every library on the AS/400.

The file is named QADBIFLD and is in the QSYS library.

So, if you want to find where a field is defined, use QUERY/400
to read QSYS/QADBIFLD and select records where QDBIFLD = 'xxxx'

Or, use run this SQL statement:
SELECT dbilib, dbifil, dbipos, dbifld, dbityp, dbifln, dbinsc,
dbitxt FROM qadbifld WHERE dbifld = 'xxxxx'