unlock iphone without erasing it

No, data is NOT gone. It is all still on the phone just like it would be on a computer. It’s there, and the question is how can I access that data on my old phone that is passcode-protected? (When I’ve forgotten the passcode.) Can anyone answer this?

If it's locked, then your data is already gone, all you can do now is restore it.

If you forgot the passcode on your iPhone, or your iPhone is disabled or unavailable -

How to reset your passcode without a computer if you have iOS 15.2 or higher.

https://support.apple.com/en-us/HT212951

How to reset your passcode using a computer.

If you forgot your iPhone passcode - Apple Support

Your data was encrypted to the passcode you used. That passcode is no longer useable to unlock your phone so your data IS gone.

You now have to wipe your phone and start over.