Adding an item to the entity/block interface

Adding an item to the entity/block interface. I can't figure out there's a block for adding an item to a specific cell, but the problem is that if I want to add a new item to an existing one in a cell, it just replaces it instead of adding it, how do I make it add an item to an existing one instead of replacing it? I just noticed it in both the block interface and the entity interface, and I can't figure out how to implement it.                                                                                                                                                                                                                                                                                                                                                        I also want to make an entity that will harvest in the garden beds, but I ran into the problem of checking crop growth. I found the age check, but as I understand it, it does not work on vanilla plants.