Instructions for compilation from multiple source files

[Solomon E via Digitalmars-d-learn]

On Monday, 10 November 2014 at 12:21:51 UTC, bearophile wrote:

> It's not in flux.
> A simple way to compile more than one file is to put them in 
> the same compilation unit (almost the same if you want to 
> create a lib):
>
> dmd a.d b.d
>
> Otherwise you can also use ddmd and let it find the module 
> dependencies by itself.
>
> Bye,
> bearophile

That's not applicable because I'm not using DMD. Also that
doesn't answer where a .o or .a file comes from, whether there's
any difference between them besides the name, and if so what.
I've compiled a lot of little examples and tests of D from single
source files, but I haven't seen any .o or .a files yet.

I was hoping there were instruction pages or documentation pages
on how to compile D projects that have multiple source files.