As you said, the creation of the connection to the cluster starts with some REST calls to get the topology of the cluster, so the protocol in this case it HTTP.
Once the connection is created the client is connected to all the nodes of the cluster using a TCP connection, and the protocol that is used to do all the core operations: get/set/delete/… is the Memcached Protocol.
When you are creating a Query to call a view, the protocol is in this case HTTP/REST.
If you really want to dive into the internals of Couchbase protocol, you can: