More and more features are getting added, so the UI has to get reorganized to accomodate for them. A lot of the changes for Android 16 are to accommodate wearables and folding phones and etc while keeping the controls more consistent between them.
Cars have also gained new features, but the steering wheel, shifter, pedals, mirrors are still in the same places since >50 years. You don't need to get a new license and re-learn how to drive every time a new model comes out because they moved the steering wheel on the ceiling to install a 32 inch LCD screen.
And my android phone still has "back", "home", and "recents" buttons in the exact same place they've been since they were literal hardware buttons. Core navigation (aka, "steering") is unchanged, no new drivers license required
Last I checked you had to flip a switch in settings to have all those buttons back like that; it hasn't been a default in stock Android for some time now.
Originally there was a 4th button, "menu". Android had a standard grid-shaped menu that apps could implement, that would pop up from the bottom of the screen.
Also "recents" wasn't its own button, it was reached by long-pressing "home". The other button was "search".
