IPhone Screen Replacement and AT&T Next?

I have an iPhone 6s, and I cracked my screen. It is under AT&T and AT&T Next. I'm not planning on upgrading any time soon, but I want to replace the screen and would rather replace it myself with an OEM part than pay $120 for the same thing. I have 2 questions: 1.) I've been told that OEM parts are the exact same thing as the original parts, so does it have all of the markings and stuff? And 2.) Would AT&T check or even care? I know that there are serial numbers on most parts in the iPhone and I assume it's matching for every part, so I was wondering if they check that and if they would care if they found out that I opened it and replaced it.

OEM means Original Equipment Manufacturer. The only person who can supply OEM parts for an Apple device is Apple, even if the part itself is manufacturer by a subcontractor. Apple only supplies it's subcontractors with replacement parts so it is unlikely that the part can be considered genuine, so if this were spotted at trade in it could well cost you!

Att won't care what screen you use but if you want a true original apple screen it needs to come directly from apple. There's a marketing technique to sell parts labeled "OEM" but that does not mean you will actually get a apple screen. It's like in the late 90s there was a country in Eurasia somewhere named "USA". Products from this country were labeled made in USA to increase sales.