Where do I link my iPhone 12 Pro Max to my iPad?

What, specifically, are you attempting to achieve by pairing an iPhone and iPad?

A direct cabled link between iOS/iPadOS devices is not possible.

Whilst it is technically possible pair devices via Bluetooth, for the majority of applications, Bluetooth pairing of an iPad and iPhone is entirely pointless. If your intention is to use Cellular Hotspot features of your iPhone, this link is optimally achieved using the native WiFi hotspot connection - this being a largely automated process. Hotspot connections over WiFi offer substantially greater throughput, range and reliability.

If your intention is to share/synchronise data between devices, if both a signed-in to iCloud with the same AppleID, then iCloud Continuity services will provide synchronisation, backup and cloud storage.

Set up iCloud on your iPhone, iPad, or iPod touch - Apple Support

https://www.imore.com/icloud

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

JEATexas wrote:

Where do I link my iPhone 12 Pro Max to my iPad?

Link it to do what exactly?