The complete installer appears in the Applications folder, just like it would if you were updating an older macOS version.
Open the /Applications folder on the Mac to find the macOS Big Sur installer application.
You’ll see a “Scanning for installer” message soon followed by a percentage indicator of the download, when finished downloading you’ll see “Install finished successfully”.
Open the Terminal application (hit command+Spacebar and type Terminal, then hit return, or launch it directly from Utilities).
Assuming you’re currently on a Mac running Big Sur (11.0 or later) here is all you need to do: The simplest and most reliable way to redownload macOS Big Sur directly from macOS Big Sur is by using the command line. Re-downloading macOS Big Sur from a macOS Big Sur Mac Not to worry, as it’s easy to download macOS Big Sur from macOS Big Sur by using the command line as we’ll show you.