How to unlock iPhone passcode as the real owner with serial number?

The only way to unlock an iPhone is with its passcode. If you do not know that passcode, the only path forward is to erase the device and restore its data from a backup.

See If you forgot your iPhone passcode or your iPhone is disabled - Apple Support

The Serial number of your iPhoine is not required for unlocking the device.

Your data is not lost if you have a backup either on iCloud or iTunes.

If you see an iPhone or iPad unavailable message or a "Security Lockout" screen

If you forgot your passcode, you can use your Apple ID and password to erase and reset your device without a computer.

Learn what to do if you see "[Device] unavailable" or "Security Lockout" on your iPhone or iPad.)

iPhone is disabled.

Following this process is the only way out if your iPhone is disabled. The steps given in this link will help you reset the passcode -> If you forgot the passcode for your iPhone, iPad, or iPod touch, or your ...

Regarding your data…

1. If you have an iCloud or iTunes latest backup, then there is nothing to worry about. If not read more...
2. You have synced your data to iCloud, then what you will get back is what you have synced to iCloud and uploaded to iCloud Drive; the information that is synced and stored in iCloud are, Contacts, Calendars, Bookmarks, Mail, Notes, Voice Memos, shared photos, iCloud Photos*, Health data, call history, and files you store in iCloud Drive. (*-If you have turned on iCloud Photos, your content is automatically stored in iCloud.)
3. You have imported your Photos and videos to Mac/PC then they are safely residing there on Mac/PC
4. You have uploaded some of it to Cloud Drives using the Files app (iCloud Drive, Google Drive, OneDrive, DropBox, Flickr, box, AmazonDrive, Adobe Creative Cloud, etc). The data remains there.
5. If you have not backed up or do not have the latest backup, nor imported nor you have synced or uploaded your data to cloud drives prior to the situation that you are in, then I am sorry, all your data is lost.

Passcode is the most basic and fundamental building block of Apple’s mobile device security.

It’s either correctly entered on the device …

… or the device and it’s onboard data are forever inaccessible. There are no “workarounds.”

HOWEVER, you DO have a not-too-painful way-forward which should — in most instances — restore continued use of the device AND recover your data from your most-recent Automatic iCloud Backup:

If you forgot your iPhone passcode or your iPhone is disabled - Apple Support