Two pods bear the same label, app: dev.

Yes, if the pods are in the same Kubernetes namespace as the object bearing the label selector.

Yes, if the pods are in the same Kubernetes namespace as the object bearing the label selector and both pods were pre-existing when the label selector was declared.

Yes, if both pods were pre-existing when the label selector was declared.

Yes, as long as all the containers in those pods are passing their livenessProbes and readinessProbes.