The reason you are not seeing the GCP Project ID directly in the system.billing.usage table is that Databricks system tables track DBU usage at the workspace, cluster, and workload level rather than underlying Google Cloud project infrastructure.

As such, Databricks workspaces in GCP are typically deployed either within a specific customer-managed VPC / GCP Project or across multiple projects and teams successfully implement showback/chargeback using a few proven strategies.

Solution 1: Workspace to Project Mapping Table (Easiest)

If your architecture utilizes specific Databricks Workspaces to specific GCP Projects (i.e., Project A hosts Workspace A, Project B hosts Workspace B), create a reference mapping table in Unity Catalog.

Create a Mapping Table and Join with System Billing Usage