This question is actually too broad...
What I really want to know is whether or not it actually chroots and, if so, how a SSH user deamon can be launched in that jail in spite of the obvious lack of the required binary/lib in the chroot.
Google is surprisingly silent on the matter. But a good reference to explain that woud be enough (however I'm not litterate enough to read and understand their C).
: I'm talking about the actual transient daemon with user priviledges that is launched upon connection by the main root OpenSSH daemon.