In the recovery window, select if you want to wipe your internal drive for a clean install; otherwise, select Install macOS Big Sur to upgrade. Follow the on-screen prompts to complete the installation. Troubleshooting & Special Cases
To create a bootable installer from a macOS Big Sur 11.2.1 DMG, you must first convert the disk image into the "Install macOS Big Sur" application. You cannot simply "burn" a DMG to a USB drive and expect it to boot on modern Macs because modern installers require the createinstallmedia tool for proper configuration. macos big sur 1121 installer dmg
Many users want a pure .dmg file. To get the : In the recovery window, select if you want
Replace MyUSB with the name of your USB drive (formatted as Mac OS Extended (Journaled) or APFS). In the recovery window