6.0-rc1 was just released. I couldn't find the iso there, only 2 assets (2 files in archive formats). How to create an ISO (if that's the assets are for)?
ISO's are available pre-built by sponsoring elementary on GitHub and can be downloaded at https://builds.elementary.io/
If you wish to build you own ISO, the instructions are avilable in the repository wiki at https://github.com/elementary/os/wiki/Building-ISO-Images