When a stone chunk gets refined, the dummy item will roll a number from 1 to 5, the amount rolled is the amount of drops you get. After that it will pick a random resource from the list based on the weight that’s assigned to it [bigger number means it’s more likely to be the one to be picked] and finally the amount of said resource will be a random number between the min and max values.
And yes, you are correct, this is a freaking lootbox. At least it doesn’t cost actual money, I guess.