These are the ingredients, each ingredient has an item icon which is one character, you can add this character to the structure to make the item show in that slot in the GUI.
There is a actions system with which you can make certain items do certain things when left clicked, here are all the available actions (opengui, play_sound, run_command & send_message):
actions:
opengui: main
play_sound:
sound: "ENTITY_ITEM_PICKUP"
volume: 1.0
pitch: 1.0
send_message:
message: You have not unlocked this yet!
run_command:
command: crafting open dirt_minion # example command
as_op: true