Yes, it's a long-lived TCP connection. It's not mandatory but the whole point of the protocol is to make one connection and send/receive all your assets over it instead of making more connections (which take time to setup and slow down page rendering).
It keeps it open as long as it is used and usually a bit past that. So if you have any traffic or are making periodic requests then it won't close the connection. If you are using SSE that can also use the same shared connection for streaming realtime updates to the client.
