How it works with D is you can do separate compilation:

    dmd -c a.d
    dmd -c b.d
    dmd a.o b.o

or do it all in one go:

    dmd a.d b.d

Over time, the latter became the preferred method. With it, the compiler generates one large .o file for a.d and b.d, more or less creating a "pre-linked" object file. This also means lots of inlining opportunities are present without needing linker support for intermodule inlining.