why do i have a line below battery/wifi bar at the upper right corner in iphone 12 mini

You can't stop it. It's built into iOS and it shows you where to swipe down to open the Control Center on your phone.

EDIT: Sorry Lawrence. I started my reply before I saw you posted the same answer.

WendyinColorado wrote:

It's strange that I never noticed it before. I hope that's all it is. The person at the AT&T store said it indicated that my phone was backing up. I don't believe that's true.

The indicator for backup is a rotating arrow in the status bar when the Control Center is open. Have you noticed the line at the bottom of the screen that is the indicator to swipe up to close an app? The line at the top is the indicator to swipe down for the Control Center.

You drag that line down to open the Control Center.

I can assure you the line is purposely there only to show where to swipe to open Control Center. And the AT&T person was completely incorrect.

No problem, reinforcement is good. And it should be reassuring to get multiple answers that agree😏