How far in advance does he have to announce it, and does the ref have to do it to begin with? Could the ref just said "Time's expired. Tied game" at the 90 minute mark and left it at that regardless of stoppages? Is it that arbitrary? And say someone is injured during the extra time, can they add more time to the game to account for it? And if a team is trailing late in a game is it to their advantage to fake an injury to get some additional time?
Like I said, total soccer novice here.