(Opens in macOS): macOS Catalina on the App Store
(assuming the app is in your Applications folder): sudo /Applications/Install\ macOS\ Catalina.app/Contents/Resources/createinstallmedia --volume /Volumes/Catalina --nointeraction macos catalina 10.15.7 download dmg
: Once the download is complete, the "Install macOS Catalina" app will be located in your Applications folder. Creating a DMG from the Installer (Opens in macOS): macOS Catalina on the App