This fork of Marlin is primarily used for building very specific, custom builds of the Marlin firmware.
The upstream branches (bugfix-2.1.x and 2.1.x) are automatically kept up to date with upstream Marlin, and the firmware is automatically built when changes are detected.
Additionally, there are many variations of the custom firmware, which you can find in the build artifacts of this repository's Actions page, ranging from different displays and UIs to direct drive, probing and much, much more.
For more information, see the build workflow, the custom build script, and the Actions page for the latest builds.