Create a VPC firewall rule in each VPC to block traffic from any source, with priority 0.

Create a VPC firewall rule in each VPC to block traffic from any source, with priority 1000.

Create two hierarchical firewall policies per department's folder with two rules in each: a high-priority rule that matches traffic from the private CIDRs assigned to the respective VPC and sets the action to allow, and another lower-priority rule that blocks traffic from any other source.

Create two hierarchical firewall policies per department's folder with two rules in each: a high-priority rule that matches traffic from the private CIDRs assigned to the respective VPC and sets the action to goto_next, and another lower-priority rule that blocks traffic from any other source.