In a best-practice Kubernetes cluster every request to the Kubernetes APIServer is authenticated and authorized. Authorization is usually implemented by the RBAC authorization module. But there are alternatives and this blog post explains how to implement advanced authorization policies via Open Policy Agent (OPA) by leveraging the Webhook authorization module.
Kubernetes is a great orchestator for containers. But it does not manage network for Pod-to-Pod communication. This is the mission of Container Network Interfaces (CNI) plugins which are a standardized way to achieve network abstraction for container clustering tools (Kubernetes, Mesos, OpenShift, etc.)
For Istio to correctly route your traffic and apply all the rules an admin has set up, it is necessary to make the traffic through an ingress-gateway. A virtual service then does the URL matching and distribution to the target services.