https://www.bleepingcomputer.com/news/microsoft/microsoft-getting-ready-to-launch-windows-10-redstone-6-preview-builds/

With Windows 10 Redstone 5 nearly feature-complete and close to entering the stabilization process, Microsoft is now preparing to begin work on the very next Windows 10 release internally codenamed Redstone 6/19H1.

According to BuildFeed, the first Windows 10 Redstone 6 build compiled by Microsoft is 18200. The Windows 10 Build 18200 was compiled on July 16 for internal testing and more such builds should be spotted online from now on.