That's exactly what it is, Plan 9 does not have $PATH. You just mount directorie...

That's exactly what it is, Plan 9 does not have $PATH. You just mount directories with binaries under /bin. When multiple binaries have the same name, the one from the last mounted directory is used, masking the others. You could have something like:

    mount /usr/bin /bin
    mount /usr/local/bin /bin
    mount ~/.local/bin /bin

Since binaries in Plan 9 are usually under additional subdirectories, conflicts are rare. For example instead of 'ping' you have 'ip/ping' which would be expanded to '/bin/ip/ping'. The same way binaries from different system architectures can live in the same filesystem, you just mount the directory for relevant architecture under '/bin'.