Several game and apps in Android the last couple of years implement what is called Immersive Mode. Basically your navigation and system bars disappear so that you can enjoy full screen mode. Google made this possible starting back in version 4.4
I used to use an app called GMD Full Screen Immersive Mode to force it when I wanted to use it but it unfortunately breaks the keyboard when you are using it. There’s also an ADB command that will trigger it but up until a few months ago everyone thought that you have to be rooted to use it. I personally stopped rooting my phones around the time I picked up my Nexus 6p, so around a year ago. It’s just not needed and I’m a huge fan of Android Pay, which is unusable if you are rooted.
XDA has an article up describing how you can enable system-wide immersive mode on demand, without root. Go check it out here.