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

Under Mono, you can write a C# Winforms program that works on both platforms. But to make that possible, the Mono team had to write their own Winforms library that essentially duplicates all of the functions of Winforms. So there is still no free lunch.