: Apple installers use time-sensitive security certificates. Older third-party ISOs may fail to install unless you manually roll back your system clock.
At least 30 GB of free storage space on your hard drive to accommodate both the installer app and the temporary disk images.
If you need the installer to create an ISO, use these official or community-vetted links:
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Unlike standard update packages downloaded directly from the Mac App Store, an ISO file is a universal disk image. It encapsulates the entire operating system installer into a single, uncompressed file that can be read across different platforms.
Use Apple's built-in createinstallmedia tool to flash the Big Sur files onto your temporary volume.
Various technology blogs and forums provide direct download links for pre-made .iso files.
If you are using a Mac to create a bootable installer rather than an ISO for a VM: Use a 16GB or larger USB drive.
