I guessing Act 2 centred around Moonrise Towers and the Underdark area connected to it, Act 3 in Baldur's Gate where we get involved in the city, meet up with people we helped in Act 1/companion quests that culminate in the City (Astarion has his master, Shadowheart has her cult, Wyll is a local "hero", Gale needs help for his Orb problem etc), with a final Act in Avernus where we battle/join the Absolute and deal with all the interesting characters in the first layer.