Why my phone is ghost touch

The ghost touch phenomenon is caused by an issue with the digitizer. When it gets damaged it can send out random signals to the processor that get interpreted as touches on the screen.

Generally the cure is to replace the digitizer. Unfortunately on virtually all modern phones the digitizer is fused to the display so you have to replace them both. iFixit has a guide showing you how to do that repair.

Samsung Galaxy M31 AMOLED Screen & Digitizer Replacement - iFixit Repair Guide

The screen itself seems to be readily available; you can get a cheaper LCD version or stick with the original OLED display (recommended). Here's one example that came up on AliExpress.

Samsung M31 Screen Replacement Original - Samsung Galaxy M31s Screen Replacement - Mobile Phone Lcd Screens - Aliexpress

Good luck; that should resolve your issue. Let us know how it turns out.