How do I transfer data from an old iPhone that I’m not able to unlock?

If you can't unlock the phone, you can't access the data. If you have an iCloud back up from the old phone, yo can restore the new phone to that back up.

If it were possible to transfer data from a locked phone to another device, think about how many iPhone thieves would be demanding enormous sums of money as a ransom to return the phone and its data to the owner.

That would be a huge security failure if you could transfer data from a phone that you could not unlock. To reset the passcode, the phone will need to be wiped. You will be able to access any data that was stored in iCloud as long as you have the Apple ID and Password of the account that is signed into the phone.