How "Google Pixel (XL)" activates the "unlimited full resolution photo and video backup" Google offers Pixel-Phone customers "Unlimited Full resolution photo and video backup". 
Does anyone know how Google handle this?    

I can imagine 3 options - but does anyone know which one matches or how it will be handled exactly?

Three thoughts:  


For instance, someone is buying the phone via the Playstore online with his google account and he wants to give it away as a gift.
Does the unlock only counts for the user who bought the phone?
or will it be activated for the person who activates/registers the phone the first time with a Google account?
or does this always counts for any further user of the Pixel-Phone? Let's say Google Drive excludes the space-calculation for any picture/video which was taken with a Pixel-Phone?
From Google Product Pixel help


  Question: ....Do you think that the unlimited photo/video backup will be tied to
  
  1)  a single account,
  
  2) any account that is signed in on the Pixel
  
  3) any photo/video with Pixel in the EXIF info or
  
  4) something else entirely?


This question is pretty similar to what you are asking


  Answer: Essentially you can only backup to one account at a time. the unlimited storage applies to that account. If you switch the account that you backup to , then the unlimited storage applies to that account.


(Emphasis Supplied)

IMO , applying this to your options


Option 1: Not Applicable - since it is not yet tired to your account, you can buy and gift it to your friend
Option 2: Yes, as long as the person continues to use it
Option 3: Counter starts afresh for new user


While this is not the official explanation from Google, it does seem logical