THC is produced beginning almost at germination. It is present on every leaf of both male and female plants. During flowering
Roughly the same amt of THC is contained in a big leaf as a bud hair (a bud hair is a modified leaf). When you're smoking bud, with all those small leaves/hair, you get higher than when smoking leaf because of the concentration of THC. A bowl load of bud can contains 50 times more THC than a single leaf.
And when you make kif/hash, the THC percentage goes up even more, because you've screened out even more of the plant material. The strongest THC concentration can be found in honey/hash oil which contains close to zero plant material.
P.S. There are other components in marijuana leaves besides THC that get you high. About 2 dozen I think. That's why a weed high is different from a hash high.