Kubernetes: API group and resources, what are their relations?-CodePudding

When an application needs to call events API to get all events of its cluster, as a programmer I may define a role like this:

apiVersion: rbac.authorization.k8s.io/v1
kind: Role
metadata:
  namespace: default
  name: pod-reader
rules:
- apiGroups: [""] # "" indicates the core API group
  resources: ["events"]
  verbs: ["list"]

What makes me confusing is the apiGroups part, I can use "events.k8s.io", or simply "", or "events.k8s.io" and "" both...

What is this apiGroups thing? I tried to read official documentation but all I found is this:

API groups make it easier to extend the Kubernetes API. The API group is specified in a REST path and in the apiVersion field of a serialized object.

There are several API groups in Kubernetes:

The core (also called legacy) group is found at REST path /api/v1. The core group is not specified as part of the apiVersion field, for example, apiVersion: v1.

The named groups are at REST path /apis/$GROUP_NAME/$VERSION and use apiVersion: $GROUP_NAME/$VERSION (for example, apiVersion: batch/v1). You can find the full list of supported API groups in the Kubernetes API reference.

This doesn't help me understand it... Why there are named groups and core groups, why I can use "" and "events.k8s.io" together?

If my resource is events, why do I need to explicitly tell K8s that there is an API group named "events.k8s.io" as if the events in "events.k8s.io" and events in resources are two different things... This question had been haunting me for days