Shutter speed issues

Hello,
I've seen people using Shutterspeed in Lightroom mobile app so I downloaded it but it didnt really look the same, like I didnt have the exposure thing and idk for some reason whenever I mess w the shutterspeed even if with a touch the whole picture turn too dark. I made a screen capture to show the whole problem, Hope I can get a solution cause I kinda need the shutterspeed.
The ISO was changing with the Shutterspeed which shouldnt cause that.
My device is: Samsung Galaxy A51.
But yeah anyways here's the screen record in the following link..
Drive link:
https://drive.google.com/file/d/1uay6uHK75J29wfX22yUNDc-Ia6koSsqD/view?usp=sharing

Cheers.