where does concentration happen in a nephron?

The path of filtrate follows this,
PCT descending loop of henle, ascending loop of henle, distal convuluted tubule, and collecting duct.
Along the path several items are absorbed and released (example: water, salt urea, glucose,). Concentration really occurs when water is reabsorbed along the distal and collecting duct

i'll write the occurrences basically:
infiltration happens from Glomerulus to Bowman Capsule
reabsorbtion happens in Proximal Tubule, Henle, Distal Tubule
secretion happens in Distal Tubule approximately 80%
concentration happens like this:
At first NH3,Hydrogen,K(potassium) which is roving inside of capillary will sent to the urine collecting duct with ATP. So in urine collecting duct is the place in a nephron which has seen the active excretion(concentration)
 hope i'm helped. i'm a high school student.and it's all my high school knowlodge.
GOOD LUCK:)