What are "legendary" items in League of Legends?
Solution 1:
From the LoL wiki: "A legendary item (or high tier item) is an item that is crafted from at least one advanced item and a recipe. It may or may not contain basic items."
Basically the best way to think about a Legendary item is they are recipes inside recipes. For example: Shurelya's Revererie requires Kindle Gem among other things. The Kindle Gem also has a recipe which requires a Ruby Crystal. So essentially the Kindle Gem has a recipe then Shurelya's Revererie also has a recipe so that makes it a legendary item.