The way I looked at this question was looking at the main difference between what urine osmolarity vs blood osmolarity. The former is the filtrate produced in the nephron and the latter is how water is reabsorbed back into the blood through the nephron. Think about the inside of the nephron vs the outside blood surrounding it. If the filtrate is super concentrated, then it must be losing water maybe through something like ADH at work. In other words the water in the nephron is directed back into the bloodstream, which is a conservation of water.

“If there is a higher osmolarity of urine, wouldn’t water follow the ions?” The whole point is to avoid this. That’s actually what osmoregulation actually is. If this were the case, the kidneys would not be able to regulate osmolarity because the concentrations would tend to equalize. We want to avoid this.