softwareupdate --fetch-full-installer --full-installer-version 10.15.7 Support Downloads (Updates Only) : Apple hosts DMG files for specific Catalina updates and combo updates
Once you have the "Install macOS Catalina.app" in your Applications folder, you can create a DMG file using Terminal or third-party tools like the gibMacOS script which is often used for creating installers on Windows. Microsoft Community Hub Creating a Bootable USB Installer Download Macos 10.15 Catalina Dmg File
But the path isn’t just technical: it’s also about caution. Because macOS installers are Apple-distributed software, integrity and authenticity matter—using tampered images risks malware, data loss, or a non-bootable system. or a non-bootable system.