Likely in the surge protector-like Q_Bert and his gang were in the first movie. Possibly also the neighborhoods to the east and west of the main game screens in Ralph's game, but that's jjust speculation.
I think the general Sugar Rush citizens (candy people, Wynnchel and Duncan, Sour Bill, etc.) will be staying either in the penthouse or in the offscreen housing. There were some concept art pieces released at the end of July that showed Felix bringing the donut cops a drink in the penthouse.
Very minor spoiler: the director confirmed that the racers are staying with Felix and Calhoun.