Traditionally, Android phones had three onscreen buttons at the bottom: Back (triangle), Home (circle) and App Overview (square).

They chose to replace those with gestures, but there was a transitional phase where there were two buttons: Back and Home.
Initially, button-navigation was the default, but you could select gesture-navigation as an option.
Currently, gesture-navigation is the default, but you can select button-navigation as an option.
I tried to get used to the gestures, but I found it difficult to use them, so I switched back to button-navigation.