The only piece of documentation on that that I know of is this article:
https://www.pulumi.com/docs/intro/concepts/organizing-stacks-projects/
We kind of do what is described in that article: we have a base project which contains the following resources:
• cluster
• operators
• controllers
In the services that we want to deploy to that base cluster and which are separate repos and Pulumi projects we have a stack reference to the base project to get the Kubernetes provider of the cluster and from there we just deploy to that cluster as you normally would.