https://istio.io/docs/ops/architecture/#mixer

Mixer is a platform-independent component. Mixer enforces access control and usage policies across the service mesh, and collects telemetry data from the Envoy proxy and other services. The proxy extracts request level attributes, and sends them to Mixer for evaluation. You can find more information on this attribute extraction and policy evaluation in our Mixer Configuration documentation.

Mixer includes a flexible plugin model. This model enables Istio to interface with a variety of host environments and infrastructure backends. Thus, Istio abstracts the Envoy proxy and Istio-managed services from these details.