Drupal Commerce code begins at the level of discussion. Before digging in to do serious module development, ideas are kicked around in IRC and the Brainstorming forums for due consideration. When the objectives are reasonably clear, a functional specification is produced to guide development, along with any necessary notes on how to implement the features or craft the APIs. These specs are themselves turned into issues in the issue tracker where development can be monitored.
When a developer is ready to get to work, he or she should chime in on the appropriate issue on d.o and get up to speed with current development.
Bugs should always be reported via the issue tracker on d.o, but feature requests can start here in the forums so they can be worked into the specification. The specification here should always be an up to date representation of what we have in code.
For historical context regarding the git migration, see this blog post.