Well, you said yourself that “i know a proxy downloads a file in lower resolution.” So if you know it’s a lower resolution how come your confused that it’s a “worse resolution”. By the way, proxy files in Shotcut are not “downloaded” they are generated and they are not “installed” they replace your original video files to work in the timeline so that they are easier to edit with.
You said you have a “pretty bad” PC so are you using Proxy together with Preview Scaling?
Also you said that you are getting “black bars on upper and lower side of video”. That sounds like letterbox bars:
But you’re saying that wasn’t on your original video and only on the proxy files? Can you share a picture of what you mean?