Download beta versions of Apple's iOS, iPadOS, macOS and tvOS firmware.
According to Wikipedia, IPSW – iPhone Software, is a file format used to install iOS, iPadOS, tvOS, HomePod, and most recently, macOS firmware for devices equipped with Apple silicon. All Apple devices share the same IPSW file format for iOS firmware and their derivatives, allowing users to flash their devices through Finder or iTunes on macOS or Windows, respectively. Users can flash Apple silicon Macs through Apple Configurator 2.
iTunes will stop the process if it detects that you have the wrong file for your device. The only thing it will cost you is another download, so please read carefully.
You aren’t downloading the file from me. You are downloading it directly from Apple, as proved by the URL it downloads from. Also, if they weren’t, iTunes would refuse to install them, as you cannot install modified IPSW files.
A release candidate build (RC) is typically the final build of a piece of software in the beta stages for developers. Typically it is the final build before a major release.
Up until before iOS/iPadOS 14.2, Apple used Golden Master (GM) but with 14.2 that changed to release candidate.