Compiling multiple source files --- each file separately, or all together?

[David Given]

On Sunday, 28 December 2025 at 21:45:59 UTC, Richard (Rikki) 
Andrew Cattermole wrote:
[...]
>> This is beyond my knowledge of D, unfortunately. Is this ever 
>> a problem in practice?
>
> Yes.
[...]
> Do not ignore import paths.
>
> If you are not compiling a module, it MUST come from -I or 
> -extI.
>
> It has implications that will result in a binary not linking.

Can you expand on both, please?