This. . . IMO Wilton Felder was probably reading the part for I Want You Back, and Freddie Washington is credited with the bass line on Forget Me Nots. Lee Sklar said that he created the bass part for Do You Know Where You Are Going To by Diana Ross. James Jamerson got dots, chord charts and sometimes jst the producer humming the song. So it depends.
In a Nathan East interview he said that you get written parts or just chord charts, but whatever turns up the best guys "leave something on the table". That means you put in something of your own into the song that makes it perk up, like a flourish or a lick. If you can do that on every song then the producers will be keen to get you on the next session.