CentOS Atomic Host is a lean operating system designed to run Docker containers, built from standard CentOS 7 RPMs, and tracking the component versions included in Red Hat Enterprise Linux Atomic Host.
When Red Hat releases a new version of RHEL Atomic Host (typically every six weeks), they upload the source for any updated packages to git.centos.org.
A CentOS project member builds the packages and puts out a call for some basic testing. If there's any package "de-branding" required (for instance: https://git.centos.org/patch/rpms!!docker/refs!heads!c7-extras), that happens at this stage. Once tested, some of the updated packages go to the centos extras repository, and others go the "rhel-atomic-rebuild" repository. The latter location contains, more or less, packages judged to be of interest only for atomic hosts.
An atomic host is defined by a JSON-formatted specification, called a treefile, that's used as input to rpm-ostree compose tree to bind "set of RPMs with configuration" to an OSTree commit.
The treefile for CentOS Atomic Host lives at https://github.com/CentOS/sig-atomic-buildscripts/blob/downstream/centos-atomic-host.json. An Atomic SIG member checks for added or removed packages in the upstream treefile, and sends a pull request to make these changes to the CentOS treefile. (https://github.com/CentOS/sig-atomic-buildscripts/pull/107)
The host is delivered in various image formats, which are built with rpm-ostree-toolbox and imagefactory according to a handful of config files (cloud and vagrant kickstarts, a TDL file, and config.ini file). These files live alongside the treefile in https://github.com/CentOS/sig-atomic-buildscripts/tree/downstream. If any changes to these files are required, an Atomic SIG member will do so by pull request.
An Atomic SIG member tests the images, and if all is well, the process runs again by a core CentOS Project member, who signs the ostree commit and pushes the produced images out to official download locations, including Amazon and Hashicorp Atlas.
An Atomic SIG member posts a release announcement to the centos-devel and atomic-announce mailing lists, and to the seven.centos.org and projectatomic.io blogs. The project member updates the CentOS wiki at https://wiki.centos.org/SpecialInterestGroup/Atomic/Download, which serves as the download page for CentOS Atomic Host.