For a long time they force re-encoded any song you added to your iTunes, it was a good day when they allowed you to control that. also when they switched to DRM free aac that was a HUGE improvement!
Apple is one of the industry drivers of aac and is doing a lot with hvec. they have good engineers.
iTunes itself is a franken beast that has always been fighting between what it is and should be and apple knows best weirdness. it always has been one of their most hated pieces of software because there have always been many things wrong each version, they improve one thing and add in another.
