Psych fact: you can only have full focus in one activity. That’s why when you are multitasking, neither of the activities is done perfectly well.
This rule also applies to having background noises/music/movies when studying: you divide your attention between studying and the background stuff and neither of them is done completely.
Also, apparently multitasking is not good for your brain! It affects your memory and attention (see source 1), and levels of stress (source 2)