LOOK you CAN to some point give priority to some head of the muscle. It is not just heavy weight. Virtually NOBODY will disagree that squats give way more outer and upper frontal quad development than heavy leg presses.
Important is: which joint is stressed most when you pull/push? Clearly leg presses stress the knee most, so it will enhance the teardrops there.
Squats place a lot of stress on hip flexors so muscles that insert into to pelvis will also develope.
Now as for pushdowns... mainly flexes at the elbow, but last part of the movement your long head comes into play as you pull back where it inserts into your rear delt
while a good bread and butter exercise it is just not brutal enough to make a significant difference if you are lacking in the outer tricep department.
Start your triceps workout with California presses.
UP: like close grip benchpress
DOWN: like French press, except, you lower to the upper chest and with much heavier weight!
It takes a while to get the motion right, but it is a killer and it improver your bench too!