Traditionally, OpenLayers has used SVN as its revision management system. At the time of writing, however, the source code repository location is hosted at GitHub. You can access the entire code repository at http://github.com/openlayers/openlayers.

Feel free to download a copy and play around with it yourself. There is a process to actually get your code permanently added to the official source code base, but it cannot hurt to download a copy of the code base and look around it yourself to figure out how it's really working!