WT Mod is Working Title mod.
Working Title is a developer that has enhanced some of the default aircraft, chiefly with avionics. There is a thing you can download on the marketplace which adds their fancy high fidelity garmin systems into the default aircraft. The AAU Update is on the marketplace
otherwise You need to opt into the beta in order to get the AAU content
https://forums.flightsimulator.com/t/read-first-welcome-to-the-aau-02-beta/591232