Apple distributes the OS package inside an application wrapper rather than a flat .dmg container. To build a portable disk image file for archiving, virtual machine deployments (like VMware or VirtualBox), or Windows deployment environments, you must convert the bundle via the command line.
: Dropped all support for 32-bit applications. Only 64-bit software will run. How to Convert the Installer App to a DMG File
Apple provides several secure ways to get the Catalina 10.15.7 installer: Download Macos Catalina 10.15.7 Dmg File
If you already have macOS Catalina installed and only need the file (useful for offline updates or multiple machines), you can download the official DMG files directly from Apple Support:
Here is everything you need to know about safely sourcing and using the Catalina DMG. Why macOS Catalina 10.15.7? Apple distributes the OS package inside an application
For the complete full installer DMG (approximately 7.71 GB):
softwareupdate --fetch-full-installer --full-installer-version 10.15.7 Use code with caution. Only 64-bit software will run
For internet-connected Macs that support newer versions, upgrading to a supported macOS release is strongly recommended for ongoing security protection.
shasum -a 256 /path/to/your/macOS\ Catalina\ 10.15.7\ 19H2.dmg