I use gestures myself. But if you tap the three lines in the navigation bar. It'll open and show you background running apps. It gives you the option to close all.... As you can see from my screenshot.
Try going to Settings and at the top in the search box type Navigation bar. Then click on the navigation bar result
and there you'll see what way your buttons are set and will help you further.
It is the 3 lines at the bottom and then a bubble appears saying close all apps. Just click it and everything closes.
I think you'll find I explained that already?