There are no genuine parts that you can buy new from third parties. And apple seems pretty clear with their software locks that salvaged parts are also not authorized. So, I believe authorized parts are the ones that come in your device from the factory, and ones they sell to you first party.
I believe some of the factory workers can occasionally take them to sell them on the side. They are OEM parts, but not sold by the manufacturer. This is the only way to get some Apple replacement parts new.
There are no genuine parts that you can buy new from third parties. And apple seems pretty clear with their software locks that salvaged parts are also not authorized. So, I believe authorized parts are the ones that come in your device from the factory, and ones they sell to you first party.
I believe some of the factory workers can occasionally take them to sell them on the side. They are OEM parts, but not sold by the manufacturer. This is the only way to get some Apple replacement parts new.