The newest macOS Catalina beta can be downloaded via the Apple Developer Center for those enrolled into 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.
It is unknown what Apple is changing or updating in any of the betas, as the support documents for each version state there are “no release notes” for the software update.
