Re "why it's concave?": Concave buttons are less likely to be pressed while sitting in your pocket - waking the screen, waking some background processes, draining the battery unnecessarily throughout the day.
To solve this problem, non-concave buttons (flat or convex) live in wells. These buttons in wells carry unnecessary aesthetic weight - lots of angles and short surfaces in a small space. Apple's hardware rarely carries such adornment - they do smooth, "authentic" surfaces.
As such, the Home button simply _is_ the well - and it's rarely pushed by accident.
To solve this problem, non-concave buttons (flat or convex) live in wells. These buttons in wells carry unnecessary aesthetic weight - lots of angles and short surfaces in a small space. Apple's hardware rarely carries such adornment - they do smooth, "authentic" surfaces.
As such, the Home button simply _is_ the well - and it's rarely pushed by accident.