Apparently I'm used to using the back button a lot. I can't find it anywhere on my Zenfone 8 which seems very weird?
Above is the screenshot from my Zenfone 5z & steps present in Zenui 6. I am not sure of options with Zenui 8 on Zenfone 8. But i hope options present in all Zenui versions are pretty same.
Go to Settings>Display>System Navigations and use Navigation Bar/Swipe up on home button option to get back button at the bottom of your screen.
Note :- If you cant find System Navigation option in display option than simply open the Settings and search System Navigations in search bar and you can set the option you desire.
So how it works in the zenfone 8 is you swipe from either the right or left edge of your screen to inward of your screen
You also always change to fixed buttons instead of gestures control.