Inventories are a place to store products. Buildings like the Warehouse have a single inventory, and all ships have two inventories: a general-purpose cargo hold and a specialized propellant tank.
Products in an inventory do not in general belong to any particular Crew. Rather, they are assets that are accessible by anyone with Remove Products permission on the inventory. If only one Crew or account has access to the inventory, that Crew or account could be said to be the owner of its contents. Putting products into a public inventory means they are available for anyone to take.
Construction sites have specialized inventories that can store exactly, and only, the supplies needed to construct that type of building.
Ship propellant tanks are specialized inventories can only store Hydrogen Propellant.