How do I fix the down volume button

This is actually a fairly simple repair for your phone. The buttons are contained in an assembly called the middle frame that is straight-forward and easy to replace. Sorry to say iFixit doesn't have a guide to that repair for you, but there's a French site called SOSav that has just what you need.

Réparation Châssis interne Galaxy A20 - Guide gratuit - SOSav.fr

As you might guess, the site is in French, but if you use it with the Chrome browser's translation feature, it becomes quite usable and their guides are well done.

With regard to replacement parts, they seem to be easy to come by; I found sellers on Amazon, eBay and AliExpress with almost no effort. The midframe you buy is supposed to match the color of the rest of your phone, but if you feel like changing things up, some of them come with the rear cover so you could easily change it to something different if you wanted to. Here's a couple of examples; they're just some of the first ones to come up so you can almost certainly find others if these aren't quite what you need.

Amazon.com: Daguys for Galaxy A20 Middle Frame Bezel Plate (Black) : Cell Phones & Accessories

For Samsung A20 A30 A40 A50 Rplace Middle Frame Plate Bezel Housing Cover Repair - eBay