One PC runs the server (npm run server); both players connect to its address. Same network → use the host's LAN IP (e.g. ws://192.168.1.20:7000/play). Over the internet → expose port 7000 with a tunnel (ngrok / cloudflare) or port-forward, and enter that URL.

Room name: both players must type the same. Bump the number (arena-2, arena-3…) for a fresh match — server keeps each room's world state on disk between sessions.