How can I connect users to another server on Java Edition?
I assume you are referring to a BungeeCord server which is a bit of an undertaking to setup. Most minigame / servers with a "hub" use BungeeCord as it allows much more players to be online consecutively- the trick? they are all connected to different servers which are then connected to each other using the bungeecord software. This is a pretty good tutorial, but be advised it was created by a server host- although the instructions are pretty universal and can be adapted to any host you use.