You hook the methods of the well-known tooltips used by build-in code and therefore typically everybody.
InformationTooltip, ItemTooltip, PopupTooltip. For a complete list download
ESOUI - The Elder Scrolls Online source code and do a text-search for "<Tooltip ".
The methods are typically
SetLink, SetBagItem. My addons use a hook technique originally created by @Garkin, a real smart dev.
Build-in code like the Inventory call these methods on a tooltip, which means your hook. There you call e.g. :AddLine with what you want to add to the tooltip.
The details depend on what you want to do.
