New User-How to Improve Video

That is incorrect, please read

slivers of… 426x240

There is really no way to get around taking something that is very low resolution like this and extracting pieces that you may then increase even larger. Even when Export > Video > Interpolation is set to its highest in Shotcut - which you should try - it will not help a lot. It is largely a case of garbage in, garbage out. The only that may help is some new technology AI up-scalers, but they are not available in Shotcut.