The .NET Bridge (netbridge.dll) is a Windows application that acts as a bridge between BPM applications and .NET Assemblies. Oracle BPM supplies this application to provide all the necessary services to catalog and use .NET components.
The .NET Bridge runs as a standalone process on the Windows server hosting the assemblies. The bridge is itself a .NET application.
On runtime, Oracle BPM connects to the .NET Bridge through a TCP port (by default 5050). Therefore, the Oracle BPM Process Execution Engine can run in a different host environment from the one where .NET Bridge is running (e.g., your BPM Engine may reside on a Solaris box and leverage .NET components running on a separate Windows server).
All components called by the bridge share the same CLI (Common Language Infrastructure) as the bridge itself since they are called using System.Reflection APIs (it follows that they share the same process).