Why my volume & Power button is not working?

Well, I can't say for sure, but generally when something like this happens we look for what the pieces have in common. In this case all three of the buttons you're having a problem with are on the same flex cable so the obvious place to start would be to replace the volume/power button flex cable.

Unfortunately, iFixit doesn't have a guide showing you how to do that repair, but I did locate one on a Spanish site called Nadie Me Llama Gallina.

Manuales / Google Pixel 3a / Side push buttons

The part itself is pretty easy to get; I usually start out checking sites like Amazon, eBay and AliExpress. Here's the first one that come up on my cursory search.

Amazon.com: Power Volume Side Button Flex Cable for Google Pixel 3A G020A G020E G020B 5.6" : Cell Phones & Accessories