It starts with a single connection.
A hypha is one fungal thread: the smallest living part of the network, and the only part that has to work. Here it is a computer joining — one command, and it reaches out to find the rest.
Nothing is opened on your side. No port, no firewall rule, no address for anyone to knock on, and nothing of yours copied anywhere. The thread goes outward, which is the whole reason a machine behind a home router or a university firewall can join at all.