How to change aspect ratio when screen mirroring to Panasonic 4K OLED TV or Apple TV 4K (3rd generation, 2022)?

when one mirror a image which is not widescreen onto a tv which is widescreen

then what should be displayed on the black bar area?

some tv's let you cut top and bottom, so it fills out the whole screen or stretch the image to fit

it's the difference between airplay mirror and airplay from inside the app say a video player app

By definition, screen Mirroring to an external TV or monitor preserves the iPad’s screen aspect ratio - this being the image that is “mirrored” to the external screen. Unless your TV/monitor will scale the display to fill the screen top-to-bottom, you’ll see a border around the mirrored screen. If re-scaled, you will instead see black bars to the left and right.

Some TVs can “stretch” the display to eliminate borders, however, stretching the image to fit the monitor display will introduce distortion.