iphone 12 won't pair with iPad 9th

Bluetooth pairing an iPhone and a Mac or iPad is not supported. 

If you are attempting to use Handoff/Continuity or AirDrop, that does involve Bluetooth but the devices are NOT paired; Bluetooth just needs to be on and the devices in range of each other. The following may help with Handoff/Continuity issues: Use Continuity to connect your Mac, iPhone, iPad, and Apple Watch - Apple Support

Also see Share files between your Mac and Bluetooth devices - Apple Support

And see the information in: https://discussions.apple.com/docs/DOC-7722

As previously stated, the two devices are not designed to Bluetooth pair with each other.

If a device is already setup and you want to use Quick Start to set it up again from another device, you will need to erase it and start over.

To erase, see the following:

To erase your device that's using iOS or iPadOS 15 or later, tap Settings > General > Transfer or Reset [Device] > Erase All Content and Settings. On iOS or iPadOS 14 or earlier, tap Settings > General > Reset > Erase All Content and Settings. This erases all data on your device. After you erase it, your device restarts and the "Hello" screen appears. 

Then see the following options for migrating: Transfer data from your previous iOS or iPadOS device to your new iPhone or iPad - Apple Support

Yes.