How can I mirror my iPhone screen onto my ipad?

I concede that WhatsApp is (always has been - and perhaps always will be ) one of the few exception. Please direct any complaint in this regard to its owner organisation - FaceBook - as this deficiency is clearly not a limitation of iOS/iPadOS.

For the small number of Apps that do not support tablet devices, this is often due to the App not being designed or intended to be used on larger screens - or sloppy programming. For the majority, Apps are available for use directly from your iPad.

There are photo/image/video transfer apps for iOS devices in the iOS App Store.

Whatever images you have on your iPhone, that are NOT on your iPad, you can copy and transfer to your iPad using one of these apps.

Do your own search in the iOS App Store for Photo transfer style apps.

There are quite a few of these.

Sorry, you can’t.

Given that both iPad and iPhone can access exactly the same resources, using [largely] the same Apps, why would you need to do so?