In the last year I've started programming in Fortran working at a research university. Most of my prior experience is in web languages like PHP or old ASP, so I'm a newbie to compile statements.
I have two different code I'm modifying.
One has an explicit statement creating .o files from modules (e.g. gfortran -c filea.f90) before creating the executable.
Another are creating the executable file directly (sometimes creating .mod files, but no .o files, e.g. gfortran -o executable filea.f90 fileb.f90 mainfile.f90).
Compiling to object files is what is called separate compilation. There are many advantages and a few drawbacks.
.hfiles). But it is good practice anyway.