They got onto a single operating certificate. They merged US Airways Dividend Miles into American AAdvantage (while following US Airways plans for a revenue-based program). They combined pilot and flight attendant seniority lists, and transitioned to a single maintenance platform.
But that’s not all of the work that had to happen. Going into 2023, a decade later, it still wasn’t done. American Airlines and US Airways had separate airport operations, and different gates, in 140 locations. Atlanta was the final station to consolidate gates and that’s just happened.