Why my volume button is working by itself

It sounds as though the volume button may have gotten bumped or otherwise damaged in some way, so my first impression would be to try replacing it. There's a small flex circuit that holds the electronic part of all three buttons, the volume up and down buttons and the power button.

Open up the phone and follow this guide for removing that flex cable. Once you have it out, verify that the button itself moves freely; if not you'll need to resolve that issue first.

Redmi Note 9 Pro - Reboots, Power On flex Replacement - iFixit Repair Guide

Then go ahead and put in a new flex cable, reassemble and reevaluate whether it's working correctly or not.