Question

how do kidneys manage to make small amounts of urine that is quite hypertonic/concentrated?

Answer 1

Reabsorption Moves Nutrients and Water Back into the Blood:

The glomerulus filters water and small solutes out of the bloodstream. The resulting filtrate contains waste, but also other substances the body needs: essential ions, glucose, amino acids, and smaller proteins. When the filtrate exits the glomerulus, it flows into a duct in the nephron called the renal tubule. As it moves, the needed substances and some water are reabsorbed through the tube wall into adjacent capillaries. This reabsorption of vital nutrients from the filtrate is the second step in urine creation. Due to high rate of reabsorption, near about 99% of filtrate is reabsorbed making the urine highly concentrated.