If the DMG contains Install macOS Catalina.app , drag it to your Applications folder. If it contains InstallESD.dmg , mount it – inside you’ll find BaseSystem.dmg (a recovery image).
softwareupdate --fetch-full-installer --full-installer-version 10.15.7 This downloads the full installer to /Applications . Then use hdiutil to create a DMG if needed. mac os catalina dmg
Apple provides legacy download links for Catalina. Search “Apple support download macOS Catalina” – you’ll get a direct App Store link or a .pkg that extracts to a DMG. How to Install Catalina from a DMG (Clean Install) Prerequisites: A 16 GB+ USB flash drive (optional) and a compatible Mac (2012–2019 models). If the DMG contains Install macOS Catalina