Download a version of macOS using softwareupdate
that will be downloaded to the /Applications
folder.
softwareupdate --fetch-full-installer --full-installer-version 10.14.6
# Create a Catalina Virtual Disk Image
hdiutil create -o /tmp/Catalina -size 8500m -volname Catalina -layout SPUD -fs HFS+J
# Mount this Image to macOS