As you all know, In Kubernetes environment, Namespace, Pod, Cluster and Container Terminologies to run the application. But when it comes to the Certificates Installation, I'd like to know whether we should install it in Pod / Namespace / Cluster level?
This question falls slightly outside of the Pega-products remit. This concerns network config and security decisions and there should be a sysadmin in your team with knowledge of K8s or cloud systems who can answer this for you. There are many consequences to how a K8s cluster and all of the corporate networking is configured. I do not think that pega users here will want to be responsible for any consequences should their advice here not be relevant for your environment.
Installing and running a K8s cluster is a sysadmin function and would not fall under a Pega Developer’s skillset.
As this is not an area that Pega forum users can offer guidance on, could I suggest that you contact someone in your organization who has this networking skillset? Someone from your IT department should be able to discuss the Certificates Installation side of things with you.