I am working on a coding project and wanted to know how multiplayer works in Java. I want to have a host computer as the server and another computer to join the host allowing multiplayer (a card game - not decided and irrelevant atm). What are some good resources and tips on how to implement this as I have no idea how to do it and my teacher (is for a school project but outside help allowed within reason) said it is very complicated.

Tried googling on how to do it but am still confused. I want a simple guide/explanation on how I could create multiplayer interaction with only two computers and no outside hardware. Would prefer not to use outside software other than Java, but could if that is the only way possible.

Thanks! TLDR: need help/links on how to create multiplayer as google results are confusing me