When I check the movie on the Shotcut screen, the results are perfect. When I see the movie on my tablet or computer screen, the results are much less beautiful, the grain size is too big. Is this normal? Is ther something to be done to correct the situation?
Are you looking at the file on your own device directly, using a reasonable quality viewer app like VLC, or are you uploading first to YouTube and viewing it there?
When you edit, are you using the tiny built-in preview screen in Shotcut, or are you using a full-sized external monitor?
Again, what are you watching? Is it the video on Youtube after you have uploaded it there or is it the video on the disk on your system. Also, what are you using to watch it? Are you using VLC or some other app. A screenshot showing the Video in the Shotcut source player and the video you find grainy would help to narrow down the problem.