No I just need it for a vm and I don’t know if a bootable usb with the app will be accepted by the vm for bootNot from Apple I'm almost certain.
Also, I'm not sure a clone DMG of an installer would work. There might be something that requires it be built from the Catalina installer app. Someone else might know.
Either way there's no official way to get a DMG.
Is this a case of not having a working Mac to create the installer from the installation app?
hdiutil create 1015.dmg -size 8g -layout SPUD -fs HFS+J
hdiutil attach 1015.dmg -noverify -nobrowse -mountpoint /Volumes/InstallCatalina
sudo /Applications/Install\ macOS\ Catalina.app/Contents/Resources/createinstallmedia --volume /Volumes/InstallCatalina
hdiutil detach /Volumes/Install\ macOS\ Catalina
hdiutil resize -sectors min 1015.dmg