Apple has made several of its softwares available for developers, notably watchOS 8.4, HomePod 15.3, tvOS 15.3, iPadOS 15.3 and iOS 15.3.
Developers can download and test out their chosen beta(s) by going to the Apple Developer Center or activating the OTA update for compatible hardware. It’s worthy to note that public betas shortly follow developer betas, and we can expect the following softwares to do the same.

Apple will be building on the new versions as it finally completed the versions of watchOS 8.3, tvOS 15.2, iPadOS 15.2 and iOS 15.2 on December 13.
Build numbers for the new HomePod software, watchOS, tvOS, iPadOS and iOS are 19K5527d, 19S5525f, 19K5527e and 19D5026g, respectively. As always, beta versions are recommended only for testing and not for primary or mission-critical devices, as it could contain bugs, issues or cause data loss.