What was the main Northern strategy for winning the war and how did the strategy take advantage of the Southern weaknesses?

The North's strategy, once reality set in, was simply a war of attrition.

The North's population far outweighed the South. Most able-bodied Southern men were called up to fight. But in the North, there were still plenty of young men who were never called up.

Not only manpower, but also in resources. The South could not have kept fighting for more than another year or two. They just didn't have the money or resources to continue the fight (depsite some gifts from some European powers).