I noticed, and I cannot explain, that some services have
NAME TYPE CLUSTER-IP EXTERNAL-IP PORT(S) AGE
web NodePort 10.104.133.249 <none> 8080:31637/TCP 12m
the PORT(S) sections looks like this8080:31637/TCP. But this service
NAME TYPE CLUSTER-IP EXTERNAL-IP PORT(S) AGE SELECTOR
grafana ClusterIP 10.109.88.206 <none> 80/TCP 84s app=grafana,release=grafanaOnly has 80/TCP. What exactly is the difference between these services? And what exactly does the port 31637 do?
Service abstract way to expose an application running on a set of Pods as a network service. With ClusterIP, services are reachable by pods/services in the cluster but cannot accessible from outside network whereas NodePort, exposed a service on a random port between range 30000-32767 which is basically map to port inside your cluster.
In your case web service is exposed as type NodePort and you can access it from outside on port 31637 whereas you could not able to access grafana service as it is not exposed to outside network.
Check this thread, What's the difference between ClusterIP, NodePort and LoadBalancer service types in Kubernetes? It will help to understand a concept and how it works.