As far as we saw, the newcomers were mostly in the Los Angeles area. Given that there were about 250,000 of them at contact, and that quarantine ended shortly before the beginning of the series, it seems like they mostly lived in LA.
That's right. One of the Pocket tie-in novels, Extreme Prejudice
by L.A. Graf, had Matt and George travel to Pittsburgh and have to deal with the different kind of prejudices and problems a Newcomer would face in a city where most people had never met one before.