> Q18: How can you get a single binary to work identically across all these diverse systems?

> Most Unix-on-Intel binary packages are already largely similar. Almost all such operating systems use the "ELF" binary 'packaging'; the various operating systems have small but significant differences, though, that make each system's ELF binary unusable on others'.

Though the scope has diminished with the decline of proprietary unixen, there is a nice bright spot with APE binaries ( https://justine.lol/ape.html ) which are in some regards even more portable since they work on Darwin (which natively uses Mach-O) and NT (which natively uses PE).