Useful links:
https://github.com/kholia/OSX-KVM
https://github.com/foxlet/macOS-Simple-KVM
https://github.com/munki/macadmin-scripts
Mojave:
#!/usr/bin/env bash
#===========================================================================
# Works only with the official image available in the Mac App Store.
# Make sure you download the official installer before running this script.
#===========================================================================
hdiutil create -o /tmp/Mojave.cdr -size 8000m -layout SPUD -fs HFS+J
hdiutil attach /tmp/Mojave.cdr.dmg -noverify -mountpoint /Volumes/install_build
sudo /Applications/Install\ macOS\ Mojave.app/Contents/Resources/createinstallmedia --volume /Volumes/install_build --nointeraction
hdiutil detach "/Volumes/Install macOS Mojave"
hdiutil convert /tmp/Mojave.cdr.dmg -format UDTO -o /tmp/Mojave.iso
mv /tmp/Mojave.iso.cdr ~/Desktop/Mojave.iso
rm /tmp/Mojave.cdr.dmg
Catalina:
#!/usr/bin/env bash
#===========================================================================
# Works only with the official image available in the Mac App Store.
# Make sure you download the official installer before running this script.
#===========================================================================
hdiutil create -o /tmp/Catalina.cdr -size 8000m -layout SPUD -fs HFS+J
hdiutil attach /tmp/Catalina.cdr.dmg -noverify -mountpoint /Volumes/install_build
sudo /Applications/Install\ macOS\ Catalina.app/Contents/Resources/createinstallmedia --volume /Volumes/install_build --nointeraction
hdiutil detach "/Volumes/Install macOS Catalina"
hdiutil convert /tmp/Catalina.cdr.dmg -format UDTO -o /tmp/Catalina.iso
mv /tmp/Catalina.iso.cdr ~/Desktop/Catalina.iso
rm /tmp/Catalina.cdr.dmg
Download the Installer App to you /Applications folder on Mac.
Once you have the ISO your halfway there!