Thanks, that's about what I was thinking.The short version as best as I remember the details, are that a DJ can record a 4 hour set in something like 45 minutes. They start talking, pick the song(s) that they want to play, can keep talking over the intro like a live DJ would. When they are done talking, they can skip over the rest of the song and get to their next voice slot. Most of the time they should be able to hear the song that is about to play or has just ended. My understanding was that many of them would record a weeks worth of shows in a few hours early in the week. There didn't/don't all do it like that though. There are times that you know that they are live because they are talking about current music news. I remember a few musicians deaths that were discussed pretty instantly.
As for the live stuff, they probably insert that as needed. I'm sure most of the DJs are available to record a snippet for a particular day, or to make a brief live announcement.