In all the documentations of X11 that I've found so far something like this is written
Communication between server and clients is done by exchanging packets over a channel. The connection is established by the client (how the client is started is not specified in the protocol). (from wikipedia)
I haven't been able to find what is this channel exactly? A network channel for example? Is it on a port? Is it a memory map? Any help is appreciated.
The phrasing of 'channel' is intentionally vague as it can be either over a local socket, a remote connection (such as SSH), a named pipe, or another method that allows client/server bidirectional communication. Which is to say, a 'channel' is simply a connection between two points that facilitates exchange of data.
When perform X11 forwarding over SSH, the channel is the SSH connection. See the
SSH
man
page for example:$ man ssh
or per the x.org documentation: