If this creature gets enough -1/-1 counters put on it to cause it to go to the graveyard, modular will put a number of +1/+1 counters on the target artifact creature equal to the number of +1/+1 counters on this creature before it left the battlefield.

Arcbound Tracker will consider spells you cast before it was on the battlefield. For example, if Arcbound Tracker is the first spell you cast in a turn, each subsequent spell you cast after it's on the battlefield will cause the last ability to trigger.