How to Turn Off Face ID to Download Apps

You can shut this off in Settings > Face ID & Passcode. Just look for the "iTunes & App Store" toggle near the top and disable it.

Keep in mind that once you do this, the phone will usually ask for your actual Apple ID password every time you download something. It can get pretty tedious typing that out for every single app update compared to just looking at the screen. If you are just trying to avoid double-clicking the side button, that is actually a different setting under Accessibility > AssistiveTouch.