The library directory (in the Figure Important parts of a UNIX filesystem, /usr/local/lib/mh) is usually separate from the MH binaries directory. Again, the location is system-dependent -- but the pathname usually has mh and lib someplace in it. The directory has text files like scan.size and scan.timely, standard template files for draft messages such as components, and a few special executable programs like mhl.
You can save the location of the MH library directory in your MH profile by using a handy trick with the mhparam command. Then, whenever you need the library directory pathname, use mhparam in backquotes:
% cp `mhparam library`/replcomps
. ...copy
system replcomps file
% cd `mhparam
library` ...now
run commands in library directory...
nmh also has an etcetera directory, like /usr/local/nmh/etc, where many of the files from the MH library directory have been moved. The files here are template draft message files, format files for scanning messages, and other non-program files. You can find the pathname of this directory with the almost-undocumented nmh command mhparam etcdir.