Urine forms as some of the blood plasma spills out, some of these spilled plasma components are taken back up into the blood, and other material is selectively sent out to be added to the urine. In other words, there are three main things happening to form urine.
glomerular filtration (blood plasma spills out) bullet tubular reabsorption (some items are selectively reabsorbed into the blood) bullet tubular secretion (some blood plasma items are selectively sent out into the tubules)
