Google's Pixel and Pixel XL runs custom version of Android with some exclusive features including a new set of navigation keys.
The Pixel key layout uses a solid triangle (back), circle (home) and square (recent apps) for the three main Android keys, with the home key having a white border around it.
You can get Pixel-Like Navigation Keys to your non-Pixel Phone with an app called Pixbar. Pixbar modifies your device's navigation bar to have solid buttons. Currently, you can only use this application on Smartphone with Stock buttons (Nexus, Motorola or Android One).
You can purchase this application just for 0.99$ on Google Play, it doesn't require a rooted Smartphone.