Download developer beta versions of Apple's iOS, iPadOS
and tvOS firmware.
Just do a Google search and you'll have all the guides you need.
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.