The "ISO" is merely the packaging; the "deep paper" lies in the Mach kernel micro-kernel design and the APFS filesystem architecture.
Macs natively use DMG (disk image) files, which support compression, encryption, and hybrid HFS+/APFS partitions. An ISO is a sector-by-sector copy of an optical disc, usually using the ISO 9660 file system. While macOS can read and even burn ISO files, an official macOS installer is not an ISO because the installer requires a bootable HFS+ or APFS volume with specific folder structures and hidden recovery partitions. Converting Install macOS.app to an ISO is possible but often results in boot failures unless done correctly (e.g., using hdiutil to create a hybrid image). macos x iso