Do not dig too deep...

I think you are reading the task incorrectly. The drugController class is to be thought of as the inventory of the Pharmacy. The Pharmacy class uses the buy method because it is buying more inventory and the Person class uses the sell method because the inventory is being sold to the Person. You should be looking at this as you are programming the logic from the inventory's perspective, not the customer's.

If it is as you say so, why then, when the sell method is run, the amount of the drug in the inventory increases:

public synchronized void sell(Drug drug, int count) {
   ...
   allDrugs.put(drug, (currentCount + count));
}

This is the very last line of the sell method. If it were as you say so, then the count in the inventory would obviously be going down, and not up. And on the flip side of the coin, the buy method should be increasing the amount in the inventory according to your explanation if the pharmacy is stocking up, but the opposite is happening:

public synchronized void buy(Drug drug, int count) {
   ...
   allDrugs.put(drug, (currentCount - count));
   ...
}

As the title says, it's best not to dig too deep, as there is no sense to be made of this.

it looks like they changed the starting code for this task. When I completed this task back in 2018, my buy and sell methods were:

public synchronized void sell(Drug drug, int count) {
   String name = Thread.currentThread().getName();
   if (!allDrugs.containsKey(drug)) {
      System.out.println("Out of stock");
   }
   Integer currentCount = allDrugs.get(drug);
   if (currentCount < count) {
      System.out.println(String.format("%s wants %d tablets of %s. In stock: %d", name, count, drug.getName(), currentCount));
   } else {
      allDrugs.put(drug, (currentCount - count));
      System.out.println(String.format("%s bought %d tablets of %s. Remaining: %d", name, count, drug.getName(), (currentCount - count)));
   }
}

public synchronized void buy(Drug drug, int count) {
   System.out.println("Purchase: " + count + " " + drug.getName());
   if (!allDrugs.containsKey(drug)) {
   allDrugs.put(drug, 0);
   }
   Integer currentCount = allDrugs.get(drug);
   allDrugs.put(drug, (currentCount + count));
}

The same code as this question from 2018: https://codegym.cc/help/1317 Now the code is the opposite. Someone at codegym either thought the old logic was backwards, or they wanted to make this task more of a challenge and have the programmer pay attention.

That could very well be the case. To me, the version you worked with makes more sense. Have a good one.