12 nodes, 200 million documents / node, very high number of searches and indexing operations. Self-hosted ES on GCP managed Kubernetes.

Lots of other options here if you don't like managing. You can use Elastic cloud, Bonsai.io, and others

A lot of places can't put their data just anywhere.

And they can pay the vendors for "bring your own cloud" or similar. If data sovereignty is important to them, then they can probably afford it. And if cost is an issue, then they wouldn't be looking at hosted solutions in the first place.

They manage it in your GCP project, so you can also make use of your commitments etc.