How do I retrieve my iPhone passcode?

If you forgot a passcode that you changed within the last 72 hours, see the "If you recently changed passcodes, use your old passcode to temporarily sign in" section of: If you see a iPhone, iPad, or Apple Vision Pro unavailable message or Security Lockout screen - Apple Support

Otherwise, see the other sections of that article

If neither of the above is an option, you will need to use a computer. See: If you forgot your iPhone passcode or your iPhone is disabled - Apple Support

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

If you are running iOS 17 or later, and you change your passcode, then forget the new one, there is a "grace period" during which you can sign in with your old passcode.

Aside from that, if you don't know the passcode, you're not getting back into the phone unless you reset and erase it. There is no mechanism to retrieve the passcode without erasing the data. Hopefully, if you have locked yourself out, you had your data synchronized to iCloud (e.g., iCloud Photos) or backed up to iCloud, so that you will be able to restore it after wiping the phone.

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