On PC, you can install old version of Windows on PC without need across to emulator, such as 98, 2000 and ME, also 95 require special patch to get recognize with recent CPU, also third party drivers are offered for anyone who install old version OS, also SATA require driver via floppy disk (I still have old floppy disk drive in my drawer) or CD if supported and ME or below will recognize the PCI Express x16 as AGP, some people are using 98 and ME to play old MS-DOS based games without need emulator.
On Mac, for outdated OS, such as Mac OS 9 require emulator and image of ROM (New for PPC emulator with Mac OS 8.5 to 9.0, old world ROM for 68k emulator, from System 7 to Mac OS 8.1), image of ROM could find from old Mac, I had tried to install Mac OS 9 via emulator and won't work due no ROM, forgot it. Power Mac G5 and latest PPC Mac (after 2003) are unable to run Mac OS 9 so natively as well, same goes to earlier OS X version if latest OS X is offered only on new Mac.
It's just interesting to compare on outdated OS that could install on PC or Mac.