When you have an item on the Merge Board, you can tap on the blue [i]-button in the upper right corner of the information box to see the merge chain of that item up to the highest level item you have merged.

The information on which items are needed to produce other items or where to discover new items is not explicitly shown in the game, it is part of the mystery of Merge Mansion. Usually if you need an item that is not produced by your current ones or cannot be merged from them, you’ll need to gain a chest to find it.

Some items you can only gain from events, such as the Hood Ornament or the Casey Sign.