black hairline horizontal stripe appears on my iphone 11

If it appears while using all apps and in the home screens, it might be a hardware failure but we, just users like you, cannot diagnose that issue remotely, sorry.

I suggest that you contact Apple Support directly, using thius link:--> Contact Apple for support and service (they have diagnostic remote tools), or make an appointment (*) and have your device inspected at an Apple Store or AASP (Apple Authorized Service Provider) among this list:--> https://locate.apple.com/choose-country-region . After checking it out, they'll give you all the available options.

Regards

Giulio

(*) You can do it:

• Online:--> Genius Bar Reservation and Apple Support Options - Apple;
• Contacting a local Apple Store or AASP among the list provided above;
• Using the "Apple Store" app, available and downloadable from AppStore.