dh_fortran_mod will be expanded to find mod files automatically from the debian/tmp directory. It will enable the installation of mod files in parallel for multiple compilers. It will install .smod files for Fortran 2018.

The fortran-mod file syntax follows dh_install: pairs of sources and optional target directories. The default directory will be $fmoddir ( /usr/lib/$multiarch/fortran/$compiler_mod_directory/) If the target directory is absolute (starts with a '/'), this directory is used in the target package. If the target does not absolute, it will be treated as a subdirectory of $fmoddir.

$compiler_mod_directory is based on the compiler module version: currently gfortran-mod-15 for gfortran-8, and flang-mod-33 for flang-7. These will be updated for incompatible compiler versions. There is a symlink in $libdir/fortran/ for the 'canonical' compiler name, linking to this, e.g.
/usr/lib/$multiarch/fortran/x86_64-linux-gnu-gfortran-8 -> gfortran-mod-15 This enables makefile fragments to construct $fmoddir as required from :

  FC:=$(shell basename $(shell readlink -f /usr/bin/gfortran))
  FMODDIR:=/usr/lib/$(shell dpkg-architecture -qDEB_HOST_MULTIARCH)/fortran/$(FC)

Its not clear what happens on module version changes in Flang/F18. While the version number signifies format changes, it appears the design is currently to be backward compatible and read previous versions. Hence symlinks for flang handling may be required.