The majority are low volatility and stay in the plumping, and a certain amount of the water that goes through the evaporators (a little under a tenth generally, I think) is flushed through (usually into the local water treatment system) with as a more concentrated solution. Some PFAS are volatile enough to ride the water vapor up and eventually fallout again.

Edit: I forgot, some types still legal in the US are volatile enough to not fallout, and instead become strong greenhouse gases.