IIRC, in the old days anyway it was the ROM that made it Apple. No longer true?
In the old days it was not only the ROM but also proprietary hardware and cabling that made it Apple. Apple's ADB cable, for example, is identical to a S-Video cable... but then it was hard to find. Apple's serial cable, needed to connect to printers, etc., was certainly proprietary. Apple also used SCSI externals... much harder to find than PC externals. Apple had it's own line of printers, HDs, etc., that worked only with Apples. Internally, the floppy drive was buttonless and had extra circuitry added that PC floppies did not have. HDs were SCSI, and RAM was finicky about make and usually had to have a check sum RAM chip added... requiring nine RAM chips while PCs only needed eight... making it more expensive.
Re the ROM: To get OS 9 and under to work on a non-Apple product, you either had to have a software image of the ROM or the actual ROM itself. I recall trying to find a legal ROM (Apple required repair shops to return ROMS that were replaced) to use in my Amiga Mac emulator. I finally bought a dead Mac from a kid for $20 and pulled the ROM myself.