I have an 8 Plus. I've had the screen replaced twice (clumsy) and the battery with non OEM parts. However, there are non OEM parts and non OEM parts...
Some non OEM parts are genuinely exactly the same as apple parts. Same factories, same bins etc, etc. So you have to trust the repairer as to which parts they use. They all know what they're doing, some are unscrupulous. Others are not.
The big difference is with screens (that I know of, there's probably more parts that like this too). Certified Apple repairers have a gizmo that matches the serial number and tells the phone it's "genuine" part. It doesn't actually have to be a genuine part, but in order to have the gizmo and use it legitimately, the repairer pays apple a fee.
You can tell a non "genuine" screen that hasn't had the serial matched with the gizmo. True Tone doesn't work. It doesn't even appear as an option in settings.