Before installing, ensure your hardware is compatible. Big Sur requires significant storage space—at least of free space for upgrades from Sierra or later, and 44.5GB for older versions. Minimum Year Supported 2015 and later MacBook Air 2013 and later MacBook Pro Late 2013 and later 2014 and later 2014 and later 2017 and later 2013 and later Source: Quora 3. Creating a Bootable USB from a DMG
While not a direct DMG, this method gives you the .app file, which you can then convert to a DMG. macos big sur dmg
Apple does not officially provide a direct DMG download link for versions newer than macOS Sierra. Instead, you must download the installer and build the DMG yourself. Before installing, ensure your hardware is compatible