Google has an internal cloud that they talk about sometimes. Borg, bigtable, spanner, etc. They run all their services on their internal cloud, including their public cloud, with the exception of Compute Engine, which is its own thing.
Compute Engine isn't really a separate thing so much as it's another thing built on the same infra. All the way down to the individual VMs, it runs on Google's production infrastructure and is scheduled by Borg (onto dedicated machine pools, although that sort of dedicated resource pooling is not unique to GCE). Once you get into VM networking things get a bit more exotic, although all of that still runs atop the same software defined network infrastructure as everything else (but, much like with Borg, it uses more features than many workloads at Google).
(I work on Compute Engine's virtualization infrastructure)
