Then run the installer again.

That night, he backed up the precious to two external drives. He knew the internet would eventually forget. Links would rot. Servers would shut down. But as long as he had that file, his old iMac would never truly die. In a world of forced obsolescence, he had downloaded a small piece of permanence.

To obtain and install macOS Catalina 10.15.7, you generally download the full installer app through official Apple channels rather than a direct DMG file. While update patches are available as DMGs, a full clean installation typically requires a bootable USB created from the official installer app. 1. Official Download Methods