The App Store downloads an application package (Install macOS High Sierra.app). A DMG is a disk image often used for archiving or for creating bootable media on Windows or Linux machines for Hackintosh builds.

After getting the .app or .dmg , check integrity:

: For older versions where the App Store may not work, Apple hosts disk images (.dmg) for manual installation. You can find these on the official Apple Support page under the "Use a web browser for older versions" section. Combo Update