It is a single directory (which exists in many different versions for each supported system, of which only the one corresponding to your architecture shall be installed in $XASTOP/vos), which contains Fortran source files (one per routine, except for service routines called only by other routines, which are included in the same source file as the caller), and eventually C source files.
Individual routines are listed in a separate section.
VOS high level routines are Fortran-callable routines, and generally are
actually Fortran routines with a very few exceptions.
Under VMS most of them call directly system libraries or services (with the exception of the memory allocation interface which uses a couple of C jacket routines).
Under Unix they call a tier of lower level C jacket routines (Fortran-callable) which in turn call C system routines (which may not be callable directly by Fortran, usually because of the underscore loader convention).
While the majority of VOS (Virtual Operating System) Unix routines are the same for all Unix flavours, there are a few routines supplied in different forms for the various supported Unix flavours.