Most of the recipe in Vanilla 1.12 can be unlocked only by getting one of the item needed in the recipe, in your inventory. However, some of them won't unlock until you do a "special" action, or if you collect another item, that isn't needed for the recipe, here's a list of all the special actions you need to do to unlock these recipe :

• Chest → By filling 9 different slots of your inventory.
• Torch → By collecting a stone pickaxe in your inventory.
• Flint & Steel → By collecting obsidian in your inventory.
• Boat (Any type) → By going into water.
• Cauldron → By collecting a water bucket in your inventory.
• Rail → By collecting a minecart in your inventory.
• Bowl → By collecting a mushroom (any type) in your inventory.
• Tripwire hook) →By collecting a string in your inventory.
• Mossy stone brick → By collecting mossy cobblestone in your inventory.

Feel free to tell me if I forgot to mention one! Hope it helps!