BungeeCord
BungeeCord, the 6th in a generation of server portal suites
...It lets players move between separate servers while experiencing them as part of the same connected environment. The project is maintained by SpigotMC and has long been used by server owners who operate lobby, minigame, survival, creative, and specialized game-mode networks. BungeeCord works as a layer between players and backend servers, forwarding connections and managing server-to-server transitions. It also provides an API that developers can use to create plugins and extend proxy behavior. Its main value is helping Minecraft communities scale beyond a single server while keeping navigation smoother for players.