Moses warned the Israelites in Deut 7:
24 He [the Lord] will give their kings into your hand, and you will wipe out their names from under heaven. No one will be able to stand up against you; you will destroy them. 25 The images of their gods you are to burn in the fire. Do not covet the silver and gold on them, and do not take it for yourselves, or you will be ensnared by it, for it is detestable to the Lord your God. 26Do not bring a detestable thing into your house or you, like it, will be set apart for destruction. Regard it as vile and utterly detest it, for it is set apart for destruction.
After they had defeated the Midianites, in Num 31:
21 Then Eleazar the priest said to the soldiers who had gone into battle, “This is what is required by the law that the Lord gave Moses: 22 Gold, silver, bronze, iron, tin, lead 23and anything else that can withstand fire must be put through the fire, and then it will be clean.
Later in Joshua 7, Achan sinned and took some of the things designated for destruction:
19 Then Joshua said to Achan, “My son, I implore you, give glory to the LORD, the God of Israel, and give praise to Him; and tell me now what you have done. Do not hide it from me.” 20So Achan answered Joshua and said, “Truly, I have sinned against the LORD, the God of Israel, and this is what I did: 21 when I saw among the spoils a beautiful robe from Shinar, two hundred shekels of silver, and a bar of gold fifty shekels in weight, then I wanted them and took them; and behold, they are hidden in the ground inside my tent, with the silver underneath.”
the cursed things designated for destruction,
Article | Noun - masculine singular
Strong's Hebrew 2764: A net, a doomed object, extermination
In Joshua 6:18, what are “the things designated for destruction”?
They were cursed things designated for destruction, objects of idolatry.