I cannot understand why the video that I shoot in 1080P at 120fps does not come out in slow motion. Do I have to physically edit video to see it in slow motion or is it supposed to record in slow motion?

It depends upon what player you use, some will correctly determine that 120 fps is too fast and play it back at 30 fps, but other players just play it at that speed. Your best bet is to import it into a decent video editor that can understand the difference.

I am using the DJI editor and even when shooting in 1080P at 120fps the video comes back in normal speed I have to physically altar the video in order to create slow-motion when shooting in this ratio