https://cloud.google.com/interconnect/docs/how-to/dedicated/using-interconnects-other-projects Using Cloud Interconnect with Shared VPC You can use Shared VPC to share your VLAN attachment in a project with other VPC networks. Choosing Shared VPC is preferable if you need to create many projects and would like to prevent individual project owners from managing their connectivity back to your on-premises network. In this scenario, the host project contains a common Shared VPC network usable by VMs in service projects. Because VMs in the service projects use this network, Service Project Admins don't need to create other VLAN attachments or Cloud Routers in the service projects. In this scenario, you must create VLAN attachments and Cloud Routers for a Cloud Interconnect connection only in the Shared VPC host project. The combination of a VLAN attachment and its associated Cloud Router are unique to a given Shared VPC network. https://cloud.google.com/network-connectivity/docs/interconnect/how-to/enabling-multiple-networks-access-same-attachment#using_with
https://cloud.google.com/vpc/docs/shared-vpc