Apple has released the release candidate (RC) developer betas of macOS Big Sur 11.3, iOS 14.5, iPadOS 14.5, tvOS 14.5, and watchOS 7.4, which means they should be available soon for users.
Registered developers can download the betas via Apple’s developer portal. Or they can be downloaded over-the-air once the proper configuration profile has been installed from the Developer Center.