My brother put passcode on iPhone 12 and forgot passcode how do I unlock without erasing ?

how do I unlock without erasing

Unfortunately.....That is not possible. It's already too late to save the data.

If the iPhone is running iOS 15.2 or higher........you can use the instructions linked just below:

How to reset your iPhone, iPad, or iPod touch when you forgot your passcode - Apple Support

If the iPhone is not running iOS 15.2 or higher........please use the instructions linked below, but note that you must use a PC or Mac.

If you forgot your iPhone passcode - Apple Support

If he has a backup of the phone, you can use the backup to restore the data on the phone during the new setup, so the phone will look just like it did before disaster struck.

If no backup is available......a painful lesson about the importance of backups has been learned.

Maybe don’t forget the passcode

You have complete instructions. See our reply above.

Tell your brother to write down the new passcode and keep it in a safe place......even if he thinks that he will never forget it.