How do I back up my iPad Air if I can't turn it on?

You'll likely be unable to back it up. However, if the device hasn't run out of power or restarted since your last backup to a computer, you may be able to backup. Connect your device and see if you can initiate a backup.

Same with iCloud. Plug it in and leave it overnight. If you've been backing up, it may do so again.

Very simple - you cannot backup an iPad if you are unable to boot the device and unlock with your passcode. If you are unable to access the iPad, the opportunity to create a backup has been lost. A backup must be created prior to needing it for recovery purposes - not after.

iOS/iPadOS devices have an encrypted file-system - the encryption key being stored in the device security chip (known as the Secure Enclave). All data remains encrypted until the device is unlocked with the Passcode; the passcode unlocks the Secure Enclave and releases the encryption key to the operating system which decrypts your data.