i'm assuming it's mostly a wireless mic system, but they don't always sound like everybody's wearing a mic, and sometimes i actually see the boom. (i.e., "beyond scared straight")
I had read an article just last week that focused on audio post-production for reality TV series. It was an interview with a seasoned audio professional. In trying to search for that article to link here, I was amazed to see how many articles there are that give plenty of detail around your question. My Google search terms: "reality tv audio". I'm not linking them here, because I don't want to imply that I've read them. Nor am I trying to be rude by saying, "just do a search." But, in this case, a search really does turn up a lot of articles on the subject.
Both, wireless lavs and boom mics. Plant mics as well. It gets quite complicated nowadays. I just read a big article in one of the IATSE sound local magazines that highlighted a mixer and his crew on some big name reality show. Kinda crazy, lots more complicated than when I was doing location sound work back in the eighties.