Whenever you make a structure spawn, you have the option to click a box [ ] Loot tables enabled?
If true, then every time the structure spawns, all the chests will have different loot. It does this to every chest, not specific ones.
Then you have a list of options of what your loot table will be based off of:
Village Blacksmith Chests
Advanced is a section where you basically edit / make your own custom loot table.