Introduction
The WebRTC architecture for desktop (Media/WebRTC/Architecture) is based on a single process model where we can directly access platform resources from the WebRTC code. B2G, however, has a split process architecture (B2G/Architecture) where the renderer/content process runs in a sandbox and has limited access to platform resources. Generally, it accesses them by doing IPC calls via https://developer.mozilla.org/en-US/docs/IPDL
The current architectural split is shown below:
In this diagram, physical devices are shown on the left and components of the browser are on the right. Note that we show a number of arrows going through the DOM/JS layer. The implication is that MediaStreams are mediated by the DOM/JS layer. I.e., JS is responsible for plumbing MediaStreams between gUM and the PeerConnection and between the PeerConnection and video/audio tags. This doesn't mean that the media actually flows through the JS, however.