Kubernetes pods are the smallest deployable units of computing that you can create and manage in Kubernetes. A pod is a group of one or more containers, with shared storage and network resources, and a specification for how to run the containers. Pods are the atomic unit on the Kubernetes platform. Pods are not permanent units in Kubernetes. They can be terminated and replaced anytime, either by the user or by the system. Pods are ephemeral and disposable entities that are created and destroyed dynamically. Pods can be scaled up or down, migrated across nodes, or rescheduled due to failures or maintenance. Pods cannot contain deployments. Deployments are higher-level resources that manage pods and provide declarative updates to them. A deployment can create multiple pods from the same template and ensure that the desired number of pods are always available. A pod can only contain containers and volumes, not other Kubernetes resources. References: Pods | Kubernetes, Viewing Pods and Nodes | Kubernetes, What is a Kubernetes pod? - Red Hat, What are Pods in Kubernetes? - Medium, Network overview | Google Kubernetes Engine (GKE) | Google Cloud.

Pods are created in a sequential order when a StatefulSet is set up. This means that for a StatefulSet with N replicas, the Pods are deployed one by one, starting from the Pod with the index 0 and ending with the Pod with the index N-1. Each Pod has to be in the Running and Ready state before the next Pod is created. This ensures that the Pods have a predictable and unique identity and can be connected to their corresponding persistent volumes. References: 

• 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?