I'm sure he still lives there, but you don't see too much of the insides of the buildings any more.
During the first season, quite a few scenes were shown in Susan and Gordon's kitchen. (Check out the very first episode! There's a groovy peace sign hanging on the wall.). They showed a number of scenes in Bob's apartment later as the 70's wore on- the 1978 special "Christmas On Sesame Street" shows how his apartment was laid out. Around this time they also started showing scenes on the roof of 123- up to and including Maria and Luis' wedding.
There was one episode, late 70's vintage- showing the basement of 123. Ernie and Bert live down there, but they showed a separate room set up like an actual basement. There was a furnace, hot water heater (which doesn't make sense, since hot water doesn't need heating!) and a washing machine.
One time Big Bird decided he was going to help with the laundry, and dumped a lot of soap in with the clothes. A short time later, the basement is four feet deep in suds. Gordon screams at Bird, "Did you put in a whole box of soap?!!!!", and Bird replies "No, that would be silly! I only put in HALF a box!"
Good clean fun brought to you by the letters F, R and the number 5.