Mirroring

Screen Mirroring between iOS/iPadOS devices is not natively supported. 

Such a feature would be largely pointless - as, with only minor exceptions, anything that your can display one one device can be accessed/displayed by the same App installed on the other device.. Additionally, using Continuity features, you can simply use Apple Handoff functions to move many tasks between devices.

Use Continuity to connect your Mac, iPhone, iPad, iPod touch and Apple Watch – Apple Support

Unfortunately mirroring from iPhone to iPad is still not available. But you can use external 3rd party apps to make this work. Apps I suggest are

ApowerMirror

TeamViewer