See Google's New Way Of Sharing Data From Your Android Phone

Google has just turned on a feature called Instant Tethering, allowing Nexus and Pixel phones and tablets to share internet connections automatically, as long as they are connected to the same Google profile.

Announced on the company's support forum, the feature is a part of the new Google Play Services (so it may not be available for everyone just yet) and can be accessed through the
Settings -->
Google -->
Instant Tethering menu.

        The feature, according to Google, is enabled by default on all compatible devices, through Play Services.

The idea is simple: the "host," which is a Pixel, Pixel XL, Nexus 6, Nexus 5X, or Nexus 6P running Android 7.1.1 and up, automatically puts out a notification to a compatible receiver — Pixel, Pixel XL, Nexus 6, Nexus 5X, Nexus 6P, Pixel C, or Nexus 9 running Android 6.0 and up — to accept an internet connection. That's it.

Such a feature has been available for some time in the Apple ecosystem, allowing iPhones and connected iPads to share cellular connections with other iPhones, iPads and Macs, using the same iCloud login.
For its part, Google says Instant Tethering is very cognizant of battery life and usage, and won't offer a phone's connection if battery life is low.

When you unlock a tablet such as the Pixel C, it will notice if there is no internet connection available, and will ask your Pixel phone if it has internet and battery life. If it does, we will give you an option to enable a secure hotspot and pair automagically, without even taking your phone out of your pocket.

When you are done, if you don't disconnect, we will notice that you stopped using your tablet and we will disconnect the hotspot for you to save your precious battery throughout the day.