How is Hypertonic Urine Produced in a Mammalian Kidney?
The process of urine concentration in mammals occurs due to the interaction of the loop of Henle and its nearby collecting tubule. When someone is dehydrated, the kidney will aim to a maximal water reabsorption.
To concentrate urine, a maximal amount of ions will be reabsorbed. This is to maximalize the concentration gradient for water. However, the main driving force for water reabsorption is the urea concentration gradient (created by the loop of Henle). Then aquaporins will allow water to flow from the collecting tubule to the nearby medulla because of the presence of the neighboring loop of Henle. It is this proximity between the loop of Henle and the collecting tubule that allows such a high efficiency for water reabsorption.

Concentrating urine is not about leaving more ions in the water, it is about reabsorbing a maximal amount of water.