Answers to your questions:
Ans 1) When you created compass-node-agent, you specified port number as 4850 and that's why the node-agent is associated with that domain3. The connection options (host, port, user) identify the agent's target domain.
And 2) Create a node agent and specify 4911 as the value for --port option. The domain need not be started to create the node agent but you need to start the domain before you start the node agent or list node agents.
As for why asadmin is not specific to a domain (the experts will soon be here with the answer...)
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