help me understand zeds passive

im a mastery level 6 zed whom still does not know how his passive on his ult works

i understand you steal ad but im not sure how its calculated and who gives it to you?? i have some questions please

1. what do the floating shurikens above enemy heads mean and how does it deicde who to give it to
2. if my mark kills the enemy does that go towards my ad?
3. i dont get what it means when someone has higher ad they give you more, does it replace the ad you gained from your last kill??