Why is my volume down button is not working?

Unfortunately there's nothing the least bit easy about replacing the volume button flex cable. The big problem is that it's soldered to the wireless charging coil so the right way to replace it is to swap out the entire coil. I had to do that same job on an iPhone X and got a coil that came separately from the volume cable. Unfortunately when I tried to solder them together I just ended up melting the flex and ruining the whole thing.

The point of that is that you really want to replace both parts as one and not try to get them separately. The bad part of that is the wireless coil is mounted underneath everything else so you pretty much have to gut the entire phone to get to it.

I haven't been able to find any guides or videos specifically for your phone for this repair. My best suggestion is to start with the frame replacement guide. There may be parts you don't need to remove but hopefully you can judge that for yourself as you work. Offhand I am sure you'll need to remove the battery and the logic board at a minimum but I can't tell you for sure what else might be in the way of taking out the charging coil / volume button assembly.

Full frame iPhone XS Max repair - Free guide - SOSav

You might be able to get some ideas once you've got the assembly exposed to where you can get to it from the guide I made for that repair on the iPhone X.

iPhone X Charging Pad / Volume Button Assembly Replacement - iFixit Repair Guide

If you decide to tackle this repair, you might consider taking lots of pictures and using them to author your own guide to replacing the volume flex. It's easy and fun and the community will appreciate you for doing it.

Create a new Guide - iFixit

Good luck; let us know what happens!