This repo contains a Java class called "TooltipItem." This class creates a simple item with no extra features except for a custom tooltip. When creating an item that has a tooltip, it should be formatted like so:
public static final Item RUBY = registerItem("ruby", new TooltipItem(new Item.Settings(), "tooltip.tutorialmod.ruby.tooltip"));
As demonstrated above, a new TooltipItem is created, passing along with it a lang file key that points towards the actual tooltip. There is likely a better way to address this problem, but this is just what I came up with. If you have any questions, please let me know. I hope this helps!