istio Multi Primary Routing - How it routes

Question

I have a multi-primary cluster single network config , with cluster1 VS routing the host traffic to the cluster2

cluster1: virtualService

    - destination:
        host: cluster2gw.istio-system.svc.cluster.local
        port:
          number: 80

I dont see any Service entries, or any other config that says how to reach cluster2. I dont see any Nodeport for the gateway in Cluster2 either for gw , but 'clusterip' only.

My understanding is the clusterIP / CIDR is not reachable from outside of a cluster. Am Trying to understand , how the traffic is getting routed to the Cluster2 in this case( particularly the data path ).

I went throught the documents from istio , but about this datapath there is no clarity. Besides the API Trust Certs / secrets.