This is the massive ~8GB to 12GB application usually grabbed from the Mac App Store. It contains the complete operating system infrastructure and is required to create a bootable USB drive or perform a clean install. Method 1: Download Catalina 10.15.7 via the Mac App Store
For machines running macOS 10.15.6, get the Standard macOS 10.15.7 Update DMG from Apple. download macos catalina 10157 dmg file install
Once completed, the download automatically stops. Close the installer window if it opens immediately. The file is safely located in your labeled as Install macOS Catalina . Method 2: Download DMG Files Directly From Apple This is the massive ~8GB to 12GB application