No it is not OS like linux it is an OS for a network of computers. Does not care what OS the nodes are runing on
it's a node.js app which you run on your machine and which serves a localhost page with a bunch of services. Not clear which, but "messages", "files" and "streaming" are all mentioned.
yes that is correct. To access the networks service you connect your browser to the localhost. So your browser is never exposed to the Internet.