Cloud Run and Eventarc is a great combination for building event-driven services with different event routing options. There are two trigger types (Audit Logs and Pub/Sub) to choose from in Eventarc. Eventarc uses Pub/Sub as its underlying transport layer and provides convenience and standardization on top of it. If you wanted to, you could skip Eventarc and read messages directly from Pub/Sub in Cloud Run.
This blog post details three ways of receiving events in Cloud Run and provides a decision framework on how to choose.