The strong safety helps against passes and runs. The safety prevents long plays. The defensive formation usually matches up with the offense. But the defense can try to do different strategies to stop the offense. The defense can move players around to cause the offense problems.
Defenses add extra lineman or backs if they think the offense is going to run or pass.
They use another set of players call special teams . To start the game there is a kickoff . When the offense cannot move the ball they punt . After a score the scoring team kicks an extra point . The offensive team may try a field goal to score points.