Let me tell you part of their secret: they sing together. Notice that they hold the m on "some" in the first "You're nobody till sommmmebody loves you." Then on the next "some", they make the m short with a tiny rest after it. And they all sing the m's for exactly the same time. Things like that happen throughout the song. When a line ends, they all stop singing at the same instant. They are listening to each other, and the result is very professional.
The Bee Gees father, Hugh Gibb, loved the Mills Brothers and often played their records in their home when his boys were growing up. They were a big inspiration for them. Thank you for that Mills Brothers!
So it's called " Knowing how to sing"! Artists of the stature we will no have anymore because in addition to being very good also had the gift of humility! Bravo!
Così si chiama "saper cantare"! Artisti di questa statura non ne avremo più poiché oltre ad essere bravissimi avevano anche il dono dell'umiltà! Bravi!