Okay, some minutes before I post this question - the update was pushed to mirror.centos.org and an announcement was published: https://lists.centos.org/pipermail/centos-announce/2019-June/023321.html
But the actually question still remains: Which steps are between 'RedHat published an update' and 'mirror.centos.org'?
At Fedora there's 'Bodhi' where Users can review/test new updates - and give positive karma for working updates. Is there something similar for Centos or are updates checked by selected audience?
In contrast to Fedora the updates for CentOS mean mostly rebuild from the upstream sources. So testing is probably only needed for the correct build but not for included updates. They were already tested by upstream.
Regards, Simon