Because the search volume for "Download macOS Catalina 10.15.7 DMG File" is high, cybercriminals flood the web with fake DMG files containing malware, adware, or ransomware like (a macOS-specific trojan).
Once you have the .app file, you can create a bootable DMG for archive purposes:
Here is everything you need to know about safely sourcing and using the Catalina DMG. Why macOS Catalina 10.15.7?
Apple still hosts Catalina installers on its CDN for legacy users. The softwareupdate --fetch-full-installer command remains functional as of this writing.
Once you have the installer app or DMG, you can create a bootable drive (minimum 16GB USB recommended): macOS Catalina dmg file download for old macbook pro
: For specific update packages (not the full OS), you can download the 10.15.7 Update (2.86 GB) or the 10.15.7 Combo Update (4.82 GB) from the Apple Support Downloads System Requirements