How do I back up an IPhone 7 onto my ipad

You cannot backup one iOS device to another. You can backup any iOS device to iTunes or iCloud.

How to back up your iPhone, iPad, and iPod touch - Apple Support

You can restore an iOS device backup to an iOS device - Restore your iPhone, iPad, or iPod touch from a backup - Apple Support

You can restore an iPhone backup to an iPad and vice versa.

What are you actually trying to do? Are you asking how to back up your phone or are you asking how to get what's on your iPhone on your iPad?