this is all very generally speaking. rap music is extremely diverse and is in no way limited to the stuff on the radio (that i don't listen to anyway).
outkast roots dr. dre dj. quick
these are just a few of the individuals who are very meticulous in their mixing process.
and all rap is NOT synthesized beats. in fact, much of it is actually sampled from previous musicians. again, those above either play much of the music themselves or hire bands.