As an old lady BTS fan from California, I can assure you they are singing/rapping their main parts. They do use a "guide" and backing track (some of which was Halsey on the first song), but their main parts they do sing live always.
As a fan, I can hear the differences in the parts and voice live.
The second song, Mic Drop, they sounded pretty winded compared to the recorded versions (there are like 3). They also rarely sing the version with the English chorus, so I was really impressed that they were able to do that today. They must have practiced a LOT as only one of them speaks English well.
They work really hard and are very good role models. They are also all very talented and continue to work on their voice. They also write a lot of their music/lyrics/rap.