"I need to know if my iPhone screen was replaced with an alternative one"
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I. Screen Issues:
"I asked them prior to making this post, they told me they didn't change it and that the screen is original just because it had no unknown part message."
Are they an AASP (Apple Authorized Service Provider)? If not, then id take this with suspicion. AASPs are training in performing this stuff. So, see if they are or are not: Explore Service Locations
II. Lines on Screens:
"As for the scratches, they were from the screen itself, not from a screen protector. The screen also has horizontal lines when a light is shined onto it and the corners of the (im guessing) backlight are 'sunken in'""
Sounds like you have a damaged screen. So, take a screenshot, seeing if you no longer get these lines when you open it in the Photos app. If not seen, then it is a hardware issue. If seen, then it is a software issue.
To Get this iPhone Serviced...
Contact Apple, and setup a Genius Bar Appointment or see an AASP (Apple Authorized Service Provider) in your local area. Be careful as to where you get this item, ask you may get an error of an unofficial screen. So, to avoid this continual message, it is best to have Apple replace a screen.
A. Contact Apple:
Or...
B. Contact an AASP:
- Go Here: Find Locations - Apple Authorized Reseller
- Click: Service & Support
- Enter: your location information
- See: if there is an AASP nearby
- Contact: an AASP that shows up, and find out more about the services that they offer to fix the iPhone.