Back in the Beatle era, their records were recorded on 4 channel tape recorders. Overdubs were minimal or nonexistent. Stacking tracks was a studio trick to add depth with sound layers. McCartney not only invented melodic bass lines with his original song, but often he would sing while playing crafted bass lines. I realize that is not impossible, but it does require an extra load of talent. Combining vocals with clever bass lines was Paul's strength. A classic example is Oh Darling (Abbey Road LP). At first glance it's a simple blues song, but note Paul's fresh bass line, then add the fact he is singing over his bass line. Now, learn that bass part, note for note, then add your voice over it...live. No loops or overdubs. How easy was that?