I don't understand why the order has to be this way? Please explain.

I had a lot of confusion about this task as well. Don't I need to know why I'm adding a yield() in order to know where to put it? And why would you use a random variable with a low probability to determine if the yield happens or not? When I did this, I thought the aim was to allow other threads to run before any of the lines in the method run, so I put Thread.yield() first.

This is what I got to pass the verification:

public synchronized void moveMoney(Account from, Account to, int amount) {

     if (RANDOM.nextInt(5000) > THRESHOLD_VALUE) {
         from.setBalance(from.getBalance() - amount);
         Thread.yield();
         to.setBalance(to.getBalance() + amount);
     }
     else {
         from.setBalance(from.getBalance() - amount);
         to.setBalance(to.getBalance() + amount);
     }
 }

yield does not take or release locks, it simply pauses the current thread execution. So yielding in the synchronized block will not let the current thread to release lock and let the other methods to enter the synchronized block. wait/notify method should be used to release the lock. So in my opposition (it might not be right), this task wants us to know that a Thread.yield() in the middle of those two operations won't impact the result we get. Because yield() won't release from, in and amount, which will be needed for the to.setBalance() operation.

from.setBalance(from.getBalance() - amount);
     if (RANDOM.nextInt(5000) > THRESHOLD_VALUE)
         Thread.yield();
 to.setBalance(to.getBalance() + amount);

https://www.geeksforgeeks.org/java-concurrency-yield-sleep-and-join-methods/

yield(): Suppose there are three threads t1, t2, and t3. Thread t1 gets the processor and starts its execution and thread t2 and t3 are in Ready/Runnable state. Completion time for thread t1 is 5 hour and completion time for t2 is 5 minutes. Since t1 will complete its execution after 5 hours, t2 has to wait for 5 hours to just finish 5 minutes job. In such scenarios where one thread is taking too much time to complete its execution, we need a way to prevent execution of a thread in between if something important is pending. yeild() helps us in doing so. yield() basically means that the thread is not doing anything particularly important and if any other threads or processes need to be run, they should run. Otherwise, the current thread will continue to run.

public synchronized void moveMoney(Account from, Account to, int amount) {
    if (RANDOM.nextInt(5000) > THRESHOLD_VALUE) {
        Thread.yield();

    }
    from.setBalance(from.getBalance() - amount);
    to.setBalance(to.getBalance() + amount);
}