Grey screen issue on used iPhone 15 Pro Max with non-original screen

The bottom of the problem is that non-genuine parts were installed on the iPhone, likely by a person that was not authorized by Apple to perform service on iPhones.

The issue may be anything from a defective battery to a defective motherboard to a defective screen, or a combination of any of these things.

As a result of the "bottom of the problem", Apple service won't even touch the phone now, so if you want the phone repaired.....by another non-authorized person using more non-genuine parts.....you can certainly try that route.

Your decision, but personally I don't understand why you haven't already returned the phone for a refund.

Good luck.

The All Too Common SAD Reality of Buying … - Apple Community

Return it for a refund from wherever you bought it.

The bottom of the problem is you have a non-Apple screen with poor specs.

And since Apple didn't replace it, some third party shop or perhaps the previous owner did, there is no way of knowing what sort of damage they may have caused by installing it improperly.