Cannabis is illegal in the Netherlands. Only the sale of small amounts of cannabis is tolerated in licensed coffeeshops. The problem with this system is that coffeeshops need to buy what they sell illegally on the black market.
No, the Netherlands grows its marijuana domestically (and exports about half of what is grown domestically).
But to be honest, it's not a huge amount. The Netherlands grows about 270,000 kg of marijuana annually. To put that into perspective, about 17,600,000 pounds are grown each year in just five US states (Washington, California, Tennessee, Kentucky, and Hawaii).