Indeed, Apple replaced the phone with new screen and bezel from the outside and we do not necessary need to bother what is in the internal as long as they work as new. However, my personal experience with Apple replacement units were sour ones.
I owned an iphone 6 and noticed that the screen has a yellow gradient from the top (obvious enough to distract when you use it in landscape mode). I went in to service center for the issue and granted a replacement, I didn think twice nor doubt the quality of the replacement. However, the replacement units were sub-standard to my opinion and it causes me to revisit the service center several more times.
On my first collection, the replacement phone has a dark patch (uneven lighting) from the top corner of the screen. i rejected the unit and service center open another replacement set on the spot and similar problems from one of the edge. I accept the third unit as it was visually fine upon checking.
My experience does not ends here. The replacement unit gave me cracking sound from between the screen and bezel under normal usage for approximately or less than 3 months. What I mean under normal usage* was that the phone is not taken any major hit or drop that have causes any form of paint chipping or dents, but it was cracking when I touch the screen on diagonal ends and you can feel it lifting too. I return to service center and complaint about the issue and again it granted me another replacement. (*to validate the issue, my retail unit which I sent in for Yellowish screen has experience a little more that this unit, it has minor dent taken from a 2 feet drop before are free of such cracking / screen-bezel detaching sound)
When I return to collected after a week. The replacement units have an obvious dark patch at the top corner as well, I showed it back to the service center personnel and I have to wait another week or 2 for their replacement unit to arrive. I have no choice but to wait.
The final phone I received are not all in all as perfect as new. The phone is visually perfect this time but when I hold the bare phone and glide the thumb across the edge of the screen toward the bezel. I felt the sharp edge of the metal instead of a complete smooth rounded edge. Apart of that, the screen and the bezel is fitted firmly but not perfectly. there is enough gaps for my hair to go in and pull my hair when I answer calls with the phone near my ear, I did not send in this time as I only realised this near end of my one year warranty and I am sick of going in and out of service center to bet on chances for a free of defect replacement.
This experience has causes me to doubt and lost trust on the QC of such replacement devices and I have to get super protective case for my normal usage to avoid a loosen screen and bezel under my normal usage.
And i still have a minor cracking sound from the screen and bezel for my current unit which is now out of warranty coverage period despite using under protective case all the time.