On the bottom of the flash enclosure are two compartments that make up the heart rate monitor. The bottom half contains two diodes that create a red light which shines through the skin of a finger. The top half has a grouping of four sensors that measure the light reflected back from a blood vessel. These measurements are translated into a frequency to determine your heart rate per minute.
Samsung got rid of the home button which meant moving the fingerprint sensor (right) to the back of the phone next to the camera. Notice the raised edges around the camera housing and fingerprint sensor. When holding the phone, the edges are barely detectable.
A consequence of moving the fingerprint sensor next to the camera are fingerprint smudges. To the naked eye, fingerprint smudges are hard to see, but up close the oils of the fingerprint reflect off the glass back. We tested taking photos with fingerprint smudges on the lens. Check out our results.
For reference, here's a wide shot of the front of the S8. Across the top is the earpiece (top center), the selfie camera and iris scanner (top right) and the barely visible iris recognition LED (top left). Not seen on the top left are the proximity sensor and LED indicator light for notifications.
One the left side of the phone are two hardware buttons. The larger button (on the left) is the volume rocker. The smaller button on the right a button that triggers Samsung's Bixby Voice software. To use it, the button must be pressed and held down like a walkie-talkie.
Here's an S8 Plus in gold. Both the S8 and S8 Plus have all glass front and back. The rear glass is painted front the inside. The "Galaxy S8+" logo is painted directly on the glass and if you look close enough, you can see the depth of the letters atop the goose bump texture of the gold paint.