shasum -a 256 Install\ macOS\ Catalina.app/Contents/SharedSupport/SharedSupport.dmg

For creating a bootable USB drive (which functions similarly to a mounted DMG), the following command is standard: sudo /Applications/Install\ macOS\ Catalina.app/Contents/Resources/createinstallmedia --volume /Volumes/MyVolume

Once finished, the file will be in your Applications folder. Method 2: Using Terminal (Fastest for Mac)