The safest way to get the installer is via the Mac App Store.
Once you have the DMG (or the Install macOS Catalina.app extracted from it), verify the SHA-1 or SHA-256 hash. macos catalina 10.15.7 dmg download
If you specifically need a bootable DMG (not the .app ), trusted Mac admin sites host them. The safest way to get the installer is via the Mac App Store
Before downloading, ensure your hardware is compatible. macOS Catalina 10.15.7 supports: macos catalina 10.15.7 dmg download
Open (Applications/Utilities/Terminal).
Resolved an issue where macOS would not automatically connect to Wi-Fi networks.