The key to fixing reactivity is to keep your dog under threshold most of the time.  ‘Under threshold’ means that the dog is not so upset that they are barking/lunging/panicking/attacking.  When a dog does go over threshold we need to get then out of that situation, settle them down for a bit, and try again. The incidents should be infrequent enough that your dog can recover between incidents if/when he or she does go over threshold.
The program I use counter conditions the dogs to the trigger, whatever that might be for that dog. Here’s one of many foundation games that we play:

Relax!

Relaxing on cue is trained without triggers around, or around dogs/people he is somewhat comfortable with. #1 Rule – if your dog is worried, he/she is allowed to stand up.  It is very important that your dog feels they are allowed to leave the situation.  Our goal is to create a relaxed dog, not one who is afraid to move – that is a core difference between positive and punishment based trainers.

Disengage

Teaching disengagement is very important for when your dog does go over threshold.  Before it will work in extreme situations this skill must be trained a lot while your dog is under threshold.

The Science:  Adrenaline and Cortisol vs Dopamine.

What do we want from our dogs? Calmness around other dogs/people/etc.  Imagine this scenario
*You can replace shock collar with prong or choke collar or any tool used to hurt/threaten/punish your dog.
Adrenaline
You’re wearing a shock collar. You look at a blue car and maybe try to kick it or yell at it. You hear a beep and feel a shock to your trachea. Every time you see a blue car this happens, repeating 10 or 20 times. The next time you see a blue car, what will your automatic response be? Your body will receive a shock of adrenaline to prepare you for the beep then shock you’ll probably stop trying to kick the car, but it isn’t because you suddenly feel calm around it. Every time you see a blue car or maybe a blue truck or maybe a blue bicycle, you’ll get a dose of adrenaline.
This is counter productive to building confidence and being calm. Even just wearing the collar will fill you with adrenaline. Wearing the collar just heading outside for a walk, no cars in sight, but anticipating you might see a blue car… more adrenaline.
Dopamine
If every time you see a blue car before you tried to kick it, instead of the shock we gave you cheesecake. This releases dopamine in your brain. Dopamine helps you be calm, improves your memory and helps you stay happy and motivated. Your environment is controlled so that those blue cars are far away to start. Every time you see a blue car you get a few bites of cheesecake, then are asked to turn away and move away from the car, earning another bite of cheesecake.
Eventually when you see a blue car your body will release dopamine anticipating the cheesecake and turning away from the car. Eventually going out for a walk will make you happy and relaxed, keen to find those blue cars to earn that shot of dopamine and more cheesecake – or maybe its steak today. You don’t know, but it’ll be good =)
And that is why we use food instead of shock collars to fix reactivity.
Extra reading:
San Francisco’s Reasons to Take Off the Prong Collar – https://www.sfspca.org/prong/myths