that's a tough one. It's in the gray area, despite IFAB's attempts to make handle ball evaluation as black-and-white as they can. As a ref, you're looking if the arm is in an unnatural position, away from the body, to make the body bigger. If the arm is near the body, it's considered in a natural position, and as long as the arm does not play the ball, it's not a hand ball if the ball hits the arm.
In the play in question, Cucurella moves to his right just as the ball is struck. As such, his left arm is the last thing to move. Cucurella is making an effort to pull the arm to his side when the ball hits his wrist.
As I mentioned above, this was pretty gray in terms of what to call. In real time, I thought it was not a hand ball. After looking at the replay, I still think it's not a hand ball. That said, if you showed the video to a 100 refs (which is what happens in ref training), and ask them to vote on whether it's a hand ball or not, I suspect it would have been somewhere between 60-40 (no hand ball) to 50-50 between the two options.
The ref was in good position (kudos to him for his movement in the entire game, as he was pretty much always in good position) and made the call. He motioned to the German players that complained about the no-call that the arm was in close.
If you're a fan of Spain, it's a clear-cut no hand ball. If you're a fan of Germany, it's a clear-cut hand ball. If you're a ref, you're probably in that 60-40 to 50-50 zone that I previously mentioned.