A lot of the major staff who made COD2-COD MW2 left after MW2, went to Respawn then made classics at Respawn, then returned again to make COD MW 2019. I think NoClip covered some of the development of that game (could've been another company tbh), but a ton of the core IW staff returned to make Modern Warfare.
I never understood the World at War love. That game was terrible. I don't understand how a series could step backwards so much from COD4 yet sell so many copies.
Treyarch is like the douche bag in college who is banging the smart hot chick AND stealing her notes yet and somehow gets better grades than her despite stealing her essay verbatim.