Wrong, Garageband detects the pressure by accelerometer. Because when you tap hard on device it produces light motion. Try to wiggle with your device during play and you receive higher "pressure". This technique works quite well. used on aquarium when user taps on the glass the sound gets louder.

iPhone/iPad has no such thing, in Unity or otherwise. I'm not certain, but I suspect Android doesn't, either.

It might not be the amount of pressure that apps like Garageband use (like how hard you hit a key changes the sound). Think on a 2d level. When you press on the screen, the harder you press the more you finger touches the screen, because it squishes on to the screen. Garageband, for example, may just be measuring how fast the radius of the touch increases, which tells it how fast you pushed down, thus simulating a sense of pressure. Now, whether or not you can get the size of the touch in unity I do not know.