in general, the UI mirrors and expands upon what's offered by the REST API, so that's runs and events and deployments and work pools etc (all the left nav stuff)
the prefect server uses a customizable messaging broker but that more on the implementation detail side, so for example you can hook up redis streams as the messaging implementation for events in prefect, but you won't see anything about redis streams in the UI, only the Prefect events that happen to be "messages" in that message bus