How to use full screen gestures in Oxygen OS on a OnePlus phone

If you're upgrading for the first time in a few years and coming from a phone that still used Android's traditional three-button bar at the bottom of the screen, you might be confused to find that it's been replaced with gesture-based navigation. Don't panic; it's not too hard to get the hang of, and it frees up some space at the bottom of your screen. OnePlus's Oxygen OS interface may be a bit different from the "stock" Android experience you'd get on, say, a Google Pixel, but the gestures work...