softwareupdate --fetch-full-installer --full-installer-version 10.15.7 Apple Support Updates

: Click the Get or Download button. This will open the Software Update pane in System Preferences .

Then run the installer again.

: Addressed a specific graphical glitch affecting the iMac (Retina 5K, 27-inch, 2020) equipped with the Radeon Pro 5700 XT. Security Features

When you search for a you are looking for a Disk Image file . Unlike the standard macOS installer (an .app bundle), a DMG is a container.

Use a tool like via Terminal to create a bootable USB, or use a script like gibMacOS from GitHub to download the DMG components directly from Apple's servers. System Compatibility Before installing, ensure your Mac is officially supported: Download macOS Catalina 10.15.7 Update - Apple Support

If you specifically need a DMG file for virtualization (like VMware or VirtualBox) or archival, you can convert the downloaded app into a DMG using Terminal: macOS Catalina dmg file download for old macbook pro