Braking at the bottom will cause less wear.
Because air resistance also has a braking effect, and it increases with your speed, squared.
So the faster you go, the more energy will be bled off from wind resistance.

Also, braking strongly in short bursts is better than letting your brakes rub continuously. That will cause them to overheat and glaze the brake pads, destroying them.

The amount of wear will be the same.

You don’t want to drag your brakes though because they will heat up and fade if you’re on a long descent and brake fade is very dangerous on steep descents.

Proper braking technique is brake right before any turns and then let off in the middle of a turn. You can brake a little after the turn also. You basically want your bike to roll as fast as it can and just control it before and after turns.

You’re gonna want to train yourself to go faster though because the technique i describe will have you descending very quickly and most people fear brake in descents because fast descents can be very scary for the untrained.

Physics says both use the same amount, just one is spread out more and the other is all at once. I don't have the formula, but basically it takes the same overall effort to stop an object regardless of if it's slowly or very fast.

Depending on the heat build up and dissipation and the length of the slope, constant gentle braking could result in loss of friction if the brakes overheat that would have a negative impact on being able to control the speed at the end of the hill and could result in warping discs brakes.

It would probably need to be a very long and steep slope to do that, but heat build up is the only thing that really impacts brake wear since the same amount of friction is needed to slow or stop.

I'm throwing out a guess, but the force required to stop an object is related to the square of velocity. You'd need to integrate force over the stopping distance to know for sure, but with that squared term going downhill it's almost certainly better to apply even force over a longer time. You're going to speed up otherwise, requiring more total force.

For a flat surface, the force is proportional to stopping distance, so the total force is the same regardless of distance, but you need to apply that total force in a smaller time, which for extremely short stoppimg distances surely results in different wear. In that case, it's probably much more complicated and there's some optimal braking distance or optimal braking curve, rather than some simple way to figure it out. I'd say simple advice is that if it's not comfortable for you, that's a good indicator that its causing more wear.

The best thing to do is to anticipate when you'll need to stop, and stop accelarating. Then you'll need far less force to brake. Every day I see people speed up to a light that just turned red, so there's no chance it will be green when they get there. Then they brake and wait 3 minutes. Just let off the gas as soon as you can tell you'll be waiting at the light. Of course be aware of people turning or whatever, but mostly there's no reason to quickly arrive at a light you're sure you'll be sitting at.