Applying overlays with kubectl apply -k and previewing with kubectl kustomize

Kustomize is usually organised as a base and one or more overlays. The base is a directory with the shared manifests and a kustomization.yaml listing them. Each overlay is another directory with its own kustomization.yaml that points at the base under resources and adds environment-specific changes: a namespace, a name prefix, more replicas, a different image tag, extra ConfigMap values. The base never needs to know which overlays exist.