Mirroring between iPad and iPhone devices is not supported.
Perhaps consider that to do so is largely pointless - as with only very limited exception of some Apps, both iPad and iPhone can directly access exactly the same material. For example, any web page that you can access or video that you can stream on your iPhone, can be similarly directly accessed from iPad.
Screen mirroring also preserves the aspect ratio of the device from which you are mirroring. As such, even if it were possible, the iPhone screen when mirrored would appear in exactly the same format.
However, with iOS/iPadOS15, it is now possible to mirror your device screen to a Mac computer that is running macOS Monterey.