How do you make custom GUIs?

Hey all, in Hypixel housing, you can do /menu to make you rcustom menu. May I ask if there is any way to make this in my own server? (Note: I think they had something called NPC or sth like that, and there was one called gui)
I tried downloading plugins into my aternos (Yes the free server) but it didn't work. Can anyone help pls? Thanks 🙂