For item info, there's preconstructed ItemTooltip. You can find examples in ZOS UI scripts, if you can get those.
Lua Code:
-- InitializeTooltip(tooltipControl, [anchorControl, anchorPoint, offsetX, offsetY])
InitializeTooltip(ItemTooltip)
ItemTooltip:SetBagItem(bagId, slotIndex)
ClearTooltip(ItemTooltip)
There's a bunch of :Set* methods that fill the tooltip with appropriate info; since addon's don't have access to most of tooltip's contents, so you can't just put anything you want in there.
Also you can check addons like
this one to see how you can add simple text lines to the bottom.