For those wanting lots of grungy details about how the PipeWire system
works, this
blog entry from Patrick Louis should be of interest.
“The session manager is the piece of software that is responsible for the policy: to find and configure devices, attach them appropriately to the graph, set and restore their properties if needed, route streams to the right device, set their volume, and more.
It can create it’s own objects in the PipeWire graph related to session management such as endpoints and links between them, a sort of abstraction on top of PipeWire nodes.

There are currently two implementations of the session manager:
pipewire-media session and WirePlumber.“

Source: LWN.net – Louis: PipeWire under the hood