How do you restore a backup from a new 2020 iPad Air to previous/older iPad Air2?

If you intend to “clone” one iPad’s state to the other, these support pages should get you started with available methods of transferring data and setup:

Set up your iPhone, iPad or iPod touch – Apple Support

Transfer data from your previous iOS device to your new iPhone, iPad, or iPod touch - Apple Support

Use Quick Start to transfer data to a new iPhone, iPad or iPod touch – Apple Support

“Transfer” is rather a misnomer. In reality, your data and setup is merely cloned to the new device. Nothing is moved or removed from the original device.

An alternative is to restore either an iCloud or iTunes backup to the new device:

Restore your iPhone, iPad, or iPod touch from a backup - Apple Support

An iCloud or iTunes backup can only be restored to a device that has the same, or later, version of iOS/iPadOS installed.

A backup cannot be restored to a device with an earlier version of the OS than was used to create the backup.