How does porting between Linux and Windows work?

Question

If a particular piece of software is made to be run on one platform and the programmer/company/whatever wants to port it to the other, what exactly is done? I mean, do they

Answer 1

this is the point of a cross-platform toolkit like qt or gtk, they provide a platform-agnostic API, which delegates to whichever platform the program is compiled for.

some companies don't use such a toolkit, and write their own (for whatever reason - could well be optimisation-related), meaning they can't just recompile their code for another os.