The Commit model has a context field which is a Repository model. The context has no relationship with an actual repository usable with git, mercurial, etc. There presumably is a mapping that would allow an implementor to match a forgefed Repository to (for instance) a Gitea repository but there is no hint in forgefed suggesting how this could be done.
In the context of fedeproxy it could just be the path. For instance:
When receiving a Commit message with context of https://fedeproxy.forge.chapril.org/dachary/myrepo, the fedeproxy implementation would take the path /dachary/myrepo and append it to the URL for the http://forge.chapril.org to know where the git repository is located.