Apple has moved on to its third developer beta round for iOS 15.1, iPadOS 15.1, tvOS 15.1, and watchOS 8.1, and has also released a fresh HomePod firmware beta.
The newest builds can be downloaded via the Apple Developer Center for those enrolled in the test program, or via an over-the-air update on devices running the beta software. Public betas typically arrive within a few days of the developer versions, via the Apple Beta Software Program website.
Apple has given the third builds of iOS and iPadOS 15.1 the number 19B5060d. A new HomePod beta is available as well.
AppleInsider, and Apple itself, strongly advise users avoid installing betas on to "mission-critical" or primary devices, due to the potential for data loss or other issues. It is instead recommended to install betas onto secondary or non-essential devices, and to ensure there are sufficient backups of important data beforehand.