sudo /Applications/Install\ macOS\ Big\ Sur.app/Contents/Resources/createinstallmedia --volume /Volumes/MyVolume
You now have a portable, bootable Mac environment. Whether you’re testing a new OS, running legacy software, or simply keeping a disaster recovery tool in your bag, this setup puts you in control.
sudo /Applications/Install\ macOS\ Catalina.app/Contents/Resources/createinstallmedia --volume /Volumes/MyVolume