I'll bet he knew the Nashville Number System.
Probably...but his “ear” was so good, he only had to hear a song once...or just an outline of what the producer or performer wanted to nail his performance.
Someone needs to make a movie (non-documentary) on the Wrecking Crew. Many (Campbell, drummer extraordinaire Hal Blaine, Tommy Tedesco, Dr. John, Leon Russell) have passed on, but there are more than few survivors (Carol Kaye, Michael Rubini, Spider Webb) who could offer up more material on those sessions and how they transpired.
Casting would be tough, but in the right hands (screen writer and director) it would be one heck of a film.