> his describe more a Single System Image [0] to me
No, Plan 9 is not a SSI OS. The idea is all resources are exposed via a single unified file oriented protocol: 9p. All devices are files which means all communication happens over fd's meaning you look at your computer like a patch bay of resources, all communicated with via read() and write(). e.g.:
[physical disk]<-->[kernel: sd(3)]-----< /dev/sdE0/
[audio card] <---->[kernel: audio(3)]--< /dev/audio
[keyboard]-------->[kernel: kbd(3)]----< /dev/kbd
Looking above it looks like Unix but with MAJOR differences. First off the disk is a directory containing partitions which are just files who's size is the partitions size. You can read or write those files as you please. Since the kernel only cares about exposing hardware as files, the file system on a partition needs to be translated to 9p. We do this with a program that is a file server which interprets e.g. a fat32 fs and serves it via 9p (dossrv(4)). Your disk based file system is just a user-space program.
And since files are the interface you can bind over them to replace them with a different service like mixfs(4). /dev/audio is like the old linux oss where only one program could open a sound card at a time. To remedy this on plan 9 you run mixfs which opens /dev/audio and then binds itself over /dev replacing /dev/audio in that namespace with a multiplexed /dev/audio from mixfs. Now you start your window manager and the children programs will see mixfs's /dev/audio instead of the kernel /dev/audio. Your programs can now play audio simultaneously without changing ANYTHING. Now compare that simplicity to the trash fire linux audio has been and continues to be with yet another audio subsystem.
Keyboard keymaps are a filter program sitting between /dev/kbd and your program. All it does is read in key codes and maps key presses according to a key map which is just a file with key->mapping lines. Again, keyboards are files so a user space file server can be a keyboard such as a GUI keyboard that binds itself over /dev/kbd.
Now all those files can be exported or imported to other machines, regardless of CPU architecture.
Unix is an OS built on top of a single machine. Plan 9 is a Unix built on top of a network. It's the closest I can get to computing nirvana where all my resources are available from any machine with simple commands that are part of the base OS which is tiny compared to the rest.