Locked IPhone 12

Hello,

If you had already setup iCloud Backups and started the automatic backups it can and must be locked to backup. If not, you would need to unlock it to set up iCloud Backups.

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

"Automatically back up with iCloud Backup

Follow these steps to set up iCloud Backup to automatically back up your device each day:

1. Make sure that iCloud Backup is turned on in Settings > [your name] > iCloud > iCloud Backup.
2. Connect your device to a power source.
3. Connect your device to a Wi-Fi network.
4. Make sure that your device's screen is locked.
5. If you get an alert says that you don't have enough iCloud storage space to complete the backup, follow the onscreen steps to buy more storage."

Manually backing up with a cable would need you to enter the iPhone passcode to trust the computer the first time, and then need the phone unlocked to see it in Finder to start the backup.

I hope this helps.

If you mean you forgot your passcode, no. It's too late to back it up now. You need to do that before something goes wrong.

No. Backups are for before disasters strike; after is too late.