The new macOS Catalina beta can be downloaded through the Software Update mechanism in the System Preferences app after installing the appropriate profile from the Developer Center.
We don't yet know what improvements the second update to macOS Catalina will bring, but it most likely focuses on performance improvements, security updates, and fixes for bugs that weren't able to be addressed in the macOS Catalina 10.15.1 update.
We didn't find any major new changes worth noting in the first three betas, but we'll update this article should anything notable be found in the fourth beta.
macOS Catalina is a major update that does away with iTunes in favor of new Music, Podcasts, and TV apps. It also nixes support for 32-bit apps, adds a new Find My app, brings a new Photos interface, and includes multiple privacy enhancements and other app refinements.
For more on everything that's new in macOS Catalina, make sure to check out our dedicated macOS Catalina roundup.