THC levels in marijuana have been measured and recorded since the 1980s, back when it was ~4%. In contrast, the marijuana we smoke today contains somewhere ~10% THC and is expected to reach ~16% within 5-10 years with advances in genetic manipulation. Some samples that have been checked have even been reported at around 30%; this isn’t just limited to domestic sources like indoor cultivation, but the increasing potency seen even in traditionally low-potency weed sources, like Mexican stress.
The areas of your brain that are affected by THC are the cannabinoid receptors, located in different parts of the brain, responsible for memory, decision-making and cognition. They also release dopamine, the euphoria-inducing hormone that makes us high. When the receptors are flooded, you get the munchies.