Each Couchbase server node includes integrated (or server-side) moxi server software. Server-side moxi processes are spawned and managed by the Couchbase cluster manager (so they're running on the same server-side nodes as the other Couchbase server processes). Couchbase knows how to do handle watch-dogging and reconfigures these server-side moxi processes automatically.
There is one "gateway" moxi server process in particular which the Couchbase cluster manager will configure to listen, by default, on port 11211. This gateway moxi server process is fully cluster aware and will be automatically, dynamically reconfigured by the Couchbase cluster manager whenever the cluster changes server node membership or health status.
The gateway moxi has an additional special responsibility of handling the "default" bucket, if there is one. (There will be a default bucket configured when you follow the normal Couchbase initial configuration wizard screens). So a client that connects to port 11211 will end up using the "default" bucket (if there is one).
The gateway moxi also has a special responsibility to handle any buckets that need SASL authentication. That is, a client can use the binary memcached protocol to connect to the gateway moxi and use SASL authentication (which is part of the binary memcached protocol) to associate the client's connection with a different bucket.
Also, as you create per-port buckets in your Couchbase cluster, the Couchbase cluster manager will automatically spawn and manage a separate moxi process or instance for every port-based bucket that you've created. For example, if you create a second bucket named "shoppingCarts" on port 11212, there will be a dedicated moxi process/instance that listens on port 11212 that provides access to that shoppingCarts bucket.