If you are looking for a "macOS Big Sur DMG" to create a bootable USB or perform a clean install, you need to be very careful. Apple does not provide direct DMG downloads for full macOS installers to the public via their website.
Open Terminal (Applications > Utilities) and paste this command: Download Macos Big Sur Dmg
Most websites offering a "Big Sur DMG" are dangerous (malware, ransomware) or corrupted files. Here is the to get the legitimate installer. The Safe Method: Terminal & Software Update The official macOS Big Sur installer is downloaded via the softwareupdate command or the App Store. It comes as a .app bundle, not a raw .dmg . If you are looking for a "macOS Big