Explanation
kubectl is a command-line tool that allows you to run commands against Kubernetes clusters1. You can use kubectl in SAP BTP, Kyma runtime for the following purposes:
To run interactive commands. kubectl supports various commands that let you interact with your cluster resources, such as creating, updating, deleting, describing, or executing them. For example, you can use kubectl create to create a resource from a file or kubectl exec to run a command in a container2.
To view pod logs. kubectl allows you to view the logs of a pod or a container in a pod using the kubectl logs command. This can help you troubleshoot issues or monitor the pod activity3.
To forward ports from remote machines to pods. kubectl enables you to forward one or more local ports to a pod using the kubectl port-forward command. This can allow you to access a pod from your local machine for debugging or testing purposes4.
References:
1: Kubernetes / Overview of kubectl
2: Kubernetes / kubectl Cheat Sheet
3: Kubernetes / Viewing logs
4: Kubernetes / Use Port Forwarding to Access Applications in a Cluster

Explanation
The API server is a component of the Kubernetes control plane that exposes the Kubernetes API. The API server is the front end for the Kubernetes control plane. The main implementation of a Kubernetes API server is kube-apiserver . kube-apiserver is designed to scale horizontally-that is, it scales by deploying more instances. You can run several instances of kube-apiserver and balance traffic between those instances. etcd Consistent and highly-available key value store used as Kubernetes' backing store for all clusterdata. If your Kubernetes cluster uses etcd as its backing store, make sure you have a back up plan for the data2.
You use etcd in Kubernetes as a backend for service delivery. It is one of the master components used for cluster management. You can run etcd in an external cluster or as a pod on your Kubernetes master. If you run it as an external cluster you can benefit from an extra layer of security and resiliency due to its isolation from the master3.
References:
Operating etcd clusters for Kubernetes
Kubernetes Components
What Is etcd and How Is it Used in Kubernetes?