there isn't really an official method to add other nav bars to android besides the one that comes with android at the bottom of the screen. however, there are two features of mondern androids which can be combined to create the same end result:
by changing the offsets of the window manager, we can create unused space on our screen which no app should use (or miss)
offsets can be changed by using adb shell commands. so enable usb debugging, install adb on your computer and plug in the usb cable. allow your computer to gain debugging access to the android device and run this command:
adb shell wm overscan 200,0,0,0
the four numbers are offsets on hte LEFT,TOP,RIGHT,BOTTOM side of your screen. however, depending on the default layout (landscape or portrait) of your tablet or phone, this might vary as the offset does not rotate.
if you screwed up, simply run
adb shell wm overscan reset
android has a permission called “draw over other apps” which can also be used to draw over the navbar or the other offset areas of the screen. so now using this feature in combination with the above set offset, we can create something like a nav bar which is placed over the offset area of the screen and therefore won't overlap any active apps.
if you want to run an entire app or widget in the offset are, there is an app called Overlays that can do just that.