Q: 6 plus need another apple id after reset

Is this your Apple ID?

If not, you need to contact the original owner and ask him to remove this iPhone from his list of devices. Otherwise, your iPhone will be a useless brick.

Find My iPhone Activation Lock: Removing a device from a previous owner’s account - Apple Support

I'm not sure I understand.

It's done by hackers, it's illegal and I highly doubt if this actually works.

Most likely. If you bought this phone second hand and if it is locked with someone else's Apple ID, you should contact the seller and ask to remove the device from their list of devices.

The information you need is available in the article from my very first answer in this thread. Read everything carefully.