Since the iPhone XS uses an OLED display the screen display flickering is very common when there are ink marks or after any drop because these screens are a lot more fragile than the older iPhones that did not use an OLED display before the iPhone x.
The digitiser (touch screen layer) sits between the glass layer and the display layer. The whole screen assembly will need to be replaced which will fix all of those problems that you have mentioned above.
Screen cables / connectors can’t go lose as they are held in with a metal cover with screws that have something called loctite on them (to stop the screws from getting loose over time from drops). Unless the phone has been opened before those screws won’t get loose by themselves.