You'll need to adjust the camera ISO and shutter speed settings of the Samsung Expert RAW app to capture long-exposure shots.
I've attached a video by the brilliant landscape photographer Michael Shainblum demonstrating how to achieve the shots.
Same with me. I'm on S21U and I don't see also the astro and the multiple exposure modes in my Expert Raw app. I saw videos in Youtube but they are all using S22U.
I also want to know the resolution here.