Tbf the only person referenced as the Underboss was Joseph "Beppy" Sasso who was possibly Junior's protege or maybe not due to the fact that if he was we would have seen more of him, it is possible that Paulie was the Underboss but Bobby was Street Boss which is the second most important position in the family meaning he would have authority over Paulie, Paulie as Underboss would pass the orders down to Patsy (who most likely took over christophers crew due to seniority) obviously paulie would be angry with taking orders from bobby because paulie's killed hundreds and bobby's killed one person