Example: GUI

# plugins/YourPlugin/menus/main.yml
title: "&0Main Menu"
size: 54
items:
  next:
    material: ARROW
    slot: 53
    name: "&aNext"
    on_left_click:
      - "opengui shop"

# plugins/YourPlugin/menus/shop.yml
title: "&aShop"
size: 27
items:
  back:
    material: ARROW
    slot: 26
    name: "&7Back"
    on_left_click:
      - "opengui main"

import io.github.chi2l3s.nextlib.api.gui.GuiManager;
import org.bukkit.entity.Player;

GuiManager manager = new GuiManager(this);
java.io.File menus = new java.io.File(getDataFolder(), "menus");
manager.loadFromFolder(menus);

getCommand("menu").setExecutor((sender, cmd, label, args) -> {
  if (sender instanceof Player p) manager.openGui(p, "main");
  return true;
});