How I can mirror my iPhone XS to iPad air2?

You are not going to be able to do that. That feature is not available at this point.

Furthermore, if you want to suggest this feature to Apple then please leave feedback here ➔ Product Feedback - Apple

Axel F.

There is no native support for mirroring iOS devices to each other.

You will need to find a 3rd party app to let you do that.