Istio Pilot provides platform-independent service discovery, and exposes an interface to configure rich L7 routing features such as label based routing across multiple service versions, fault injection, timeouts, retries, circuit breakers. It translates these configurations into sidecar-specific configuration and dynamically reconfigures the sidecars in the service mesh data plane. Platform-specific eccentricities are abstracted and a simplified service discovery interface is presented to the sidecars based on the Envoy data plane API.
Please see Istio's traffic management concepts to learn more about the design of Pilot and the capabilities it provides.
Istio Pilot design gives an architectural overview of its internal components - cluster platform abstractions, service model, and the proxy controllers.