As described by its creator meetecho, janus is a general purpose WebRTC endpoint that browsers can interact with, and different modules can determine what should be done with the media. You can do, text messaging, video conferencing, streaming and more other things with janus. Also if there is no such module to do stuff what you are looking, you can expand janus by yourself.
When we looking into Janus, that architecture can be conceived as modular architecture. There are two main components called as janus core and the pluggable modules. Janus core take care about everything related to WebRTC and server-side plugins to provide specific functionality in a way that is transparent to WebRTC, and as such independent from the web application.
When we goes for complex multimedia scenarios with Janus WebRTC, we can use this plugins as bricks and use them together to build what we want. Below image explain how we can use multiple plugins together. ( image source :- Janus Workshop )
This blog gives brief idea about how Janus architecture works and, how we can use that architecture to build our complex multimedia solutions. If you like to read about further visit Janus.