When we create a repository on GitHub, we can always access it with the address "firstname.lastname@example.org:user/repo" but how does it work behind the scene?
...I mean, there is only one Unix account: "git" while several users use it with different keys and different directories...
I saw in another Server Fault post (How can you do dynamic, key-based SSH similar to GitHub?) it may use the command option, but how is it secured? (How does it restrict the user to the folder he has access to?)
Also, how is it managed? Does it use only one authorized_keys-file or is there another trick?