You will attach a new Dedicated Interconnect for multiple service projects (Sales, Marketing, IT) that use a host project. In which project's VPC should you create the Cloud Router?

You should create the Cloud Router in the VPC network of the Host Project. In a Shared VPC setup, the Host Project owns the VPC network and shares it with multiple Service Projects (Sales, Marketing, IT). Dedicated Interconnects, which provide a direct physical connection to Google's network, are attached to a VPC network. Therefore, the Cloud Router, which establishes BGP sessions over the Interconnect, must reside within the Host Project's VPC network to enable connectivity for all associated Service Projects. Creating Cloud Routers in individual Service Projects is incorrect because they don't own the shared VPC network. Creating them in all projects or only the IT project would lead to an unmanageable or incorrect configuration for Shared VPC.