Apple has never officially distributed Catalina as a standalone catalina.dmg file. Any website offering a direct .dmg link is hosting a repackaged, third-party image.
Searching for a "verified" direct download of a macOS Catalina DMG macos catalina dmg direct download verified
hdiutil attach ~/Downloads/catalina_unknown.dmg Apple has never officially distributed Catalina as a
| Version | Build | File Size | Direct URL | |---------|-------|-----------|-------------| | (Latest) | 19H15 | ~8.2 GB | Download from Apple | | 10.15.7 Supplemental | 19H15 | ~8.2 GB | Mirror (Apple) | How to Verify Integrity If you download a
: A command-line script that parses Apple's software update catalogs to download the required components for a full installer. How to Verify Integrity If you download a DMG from a third-party site, you verify its checksum to ensure it hasn't been tampered with. macOS Catalina - App Store - Apple