Both.

In reality almost all of Google is hosted on Borg. But I worked briefly on a project that was heavily interlinked between the two. I believe there's been more efforts to move things into GCP.

Afaik even “google cloud” itself runs atop Borg.

Where else would it run?

G runs lots of small things on cloud but a company that is willing to fund 0.1% global efficiency efforts is never going to sacrifice far larger efficiency regressions just for the aesthetic pleasure of dogfooding cloud for major services like websearch.

You can do some stuff on cloud; for example, for a few minor projects where we needed to make some web pages, we used GCP. However, it wasn't well integrated with internal, it was effectively just a web site like any other.

I believe there had been multiple waves of attempting to get people to move to cloud, but at the time, there was no coherent strategy to move large parts of the tech stack to cloud. Unfortunately with any sufficiently complex production service that runs in borg, untangling the web of dependencies is usually too hard.

Other things work great in the cloud; if I was a Google Researcher working on public data and publishing papers, I'd intentionally use Google Cloud (with access to VMs, CPUs, and GPUs, but not any internal data like the web page crawls) with Jax, Keras, TF, etc.