If you manage to obtain the DMG file, you can create a bootable installer using tools like DiskMaker X or Terminal commands.
While the App Store downloads a .app file (found in your Applications folder), you can easily convert this into a bootable USB or a DMG file for clean installations: from the link above. Connect a USB drive (at least 16GB) to your Mac. Download Macos 10.15 Catalina Dmg File
The search for "macOS Catalina DMG" is fraught with security risks. Because Apple does not host these files on a public FTP server for direct download, third-party repositories and tech blogs fill the void. This environment is ripe for the distribution of adware, malware, and corrupted files. Unverified download sites often bundle the desired DMG with unwanted software or disguise malicious payloads as the installer. Furthermore, files hosted on third-party sites may have been modified. Verifying the integrity of a downloaded macOS installer requires checking the SHA-1 hash against known good values, a step many casual users skip. Thus, the search for a legacy OS often exposes users to contemporary cybersecurity threats. If you manage to obtain the DMG file,