Hip hop is fairly unique because in a lot of cases they are the same person but from what I can tell in most other genres they will have a band eg: Chilli Peppers creating the song, playing the actual instruments etc and then they have Rick Rubin guiding them, making decisions about song structure or which take to use and guiding them to play at their best but he doesn't actually play anything in the song.
A lot of times in hip hop tho, a rapper will just get a beat and spit on it with no direction from the person who made the beat in the first place so it falls on the rapper themselves or maybe the engineer or something to guide the process. Sometimes that ends badly