Device doesn't appear to be what it should be

I wasn’t sure where to look for this so I figured this would be a good start. I recently bought a phone from someone. It was delivered brand new in the box, still wrapped in the plastic you have to tear off. It was supposed to be an s9+. After opening it up I noticed the phone is extremely laggy and doesn’t respond well to anything, the fingerprint sensor doesn’t work, and the camera was a little blurry. So since I have some background in tech I decided to take it apart a little. First thing I noticed there was the back was a cheap plastic and not glass. After getting the back off and comparing it to what an actual s9+ looks like I can easily tell it’s not the same. Motherboard is completely different, battery takes up the entire bottom 2/3 of the phone. On top of that there are just simple things that are extremely obvious to see it’s not what it should be; it starts with android version 7.0 instead of android 8.0 Oreo like the phone was made factory new with, and secure apps like my work app for my schedule told me the phone wasn’t secure. I’m trying to figure out what kind of motherboard is here but I can’t seem to find anything about it, the model says “I8128_MB_V5.0” If anyone could tell me what that is or what I should do with this please let me know.

I sadly don’t have any way to take any pictures so you’ll have to bear with me for a while until I can get some of it.