>Point 1: I do not want my program to only run on only one OS, or to require custom code to make it multi-platform.

To run on only one OS at build time? I usually just set up cross-compilers from linux if I am making cross-platform C/C++ code.

>Point 2: What if there's no OS?

You can use a system like bitbake I think.

Which Linux distribution has packages for macOS, Windows, and Android?