Helm Deployment vs Service
The "service" creates a persistent IP address in your cluster which is how everything else connects it. The Deployment creates a ReplicaSet, which creates a Pod, and this Pod is the backend for that service. There can be more than 1 pod, in which case the service load balances, and these pods can change over time, change IP's, but your service remains constant.
Think of the service as a load balancer which points to your pods. It's analogous to interfaces and implementations. The service is like an interface, which is backed by the pods, the impementations.
The mapping is m:n. You can have multiple services backed by a single pod, or multiple pods backing a single service.