OpenWebRTC browser extensions communicate with the daemon via its local web server. Before any page load, extensions fetch JavaScript from the daemon containing WebRTC functionality and injects it in to the current context. The daemon runs in a separate process in the background.
This repository is a simple browser extension that does that for Safari.
You can find details about installation and running on the OpenWebRTC wiki but you can download the plugin here.
The code that gets injected, for the suspicious of you out there, is this.
The result is WebRTC in Safari:
If you want to modify the extension you need to use the Extension Builder in Safari: