You must obtain the official installer application. It is typically around . Create a bootable installer for macOS - Apple Support (IN)
Before starting, verify that your Mac model is officially supported by Apple. Big Sur (macOS 11) dropped support for several older models released before 2013. 2015 or later. MacBook Air: 2013 or later. MacBook Pro: Late 2013 or later. Mac mini: 2014 or later. iMac: 2014 or later. iMac Pro: 2017 or later. Mac Pro: 2013 or later. 2. Download the macOS Big Sur Installer
To create a bootable USB macOS installer and ensure Big Sur compatibility, you must first download the full installer and then use the application to write the bootable image to a flash drive . This process is essential for performing clean installations, repairing systems, or updating multiple Macs without redundant downloads. 1. Compatibility Check for macOS Big Sur