Why does this black line appear?

Hello SIRAFIL,

From your post it appears that you have a concern about a black line appearing on the screen.

We would start by power cycling the iPhone using these instructions: Restart your iPhone - Apple Support. While simple, this can refresh the iOS, apps and connections. 

If the issue remains, we would recommend having the device examined by a service Advisor. You can find an Apple Authorized Service Provider using the Apple Support App on any iOS device. You can also find them from here: Apple - Find Locations

Cheers.

It is highly unlikely that it is a defect in the screen, unless you see it in every Home screen page and every app that you use. More likely it is the virtual home button showing through the translucent background of the control dialog that is displayed.

That line shows where you can swipe up to close the current app.

That looks like the Virtual Home button that FaceID iPhones have (think white line) but it's blurred out because you have the Control Center open currently. (Or Dark Mode Enabled?)