I don't use tiktok and haven't seen the videos so this is just a guess, but my first thought is they were doing the old hollywood trick of showing bare shoulder

I don't use tiktok and haven't seen the videos so this is just a guess, but my first thought is they were doing the old hollywood trick of showing bare shoulders with the rest of their body out of frame, maybe covered with a bubble bath, to imply nudity without actually being nude.