Apple does not provide a direct standalone DMG file for the full installer on its website. Instead, they provide a link that opens the .
: Once finished, the file named "Install macOS Catalina" will appear in your Applications folder . How to Create a DMG from the Installer macos catalina 10.15.7 download dmg
If you specifically need a .dmg file for a virtual machine (like VMware or VirtualBox) or for archiving, you can convert the installer app using : macOS Catalina dmg file download for old macbook pro Apple does not provide a direct standalone DMG