There are two ways in which Kubernetes can discover a Service:

Environment Variables: The kubelet service running on the node where your Pod runs are responsible for setting up environment variables for each active service in the format {SVCNAME}_SERVICE_HOST & {SVCNAME}_SERVICE_PORT. Do note that you must create the Service before the client Pods come into existence. Otherwise, those client Pods won't have their environment variables populated.

DNS: The DNS service is implemented as a Kubernetes service that maps to one or more DNS server pods which are scheduled just like any other pod. Pods in the cluster are configured to use the DNS service, with a DNS search list that includes the pod’s own namespace and the cluster’s default domain. A cluster-aware DNS server, such as CoreDNS, watches the Kubernetes API for new Services and creates a set of DNS records for each one. If DNS has been enabled throughout your cluster then all Pods should automatically be able to resolve Services by their DNS name. The Kubernetes DNS server is the only way to access ExternalName Services. ( Refer - DNS for Services and Pods )