Google has recently introduced Instant Tethering for Nexus and Pixel devices running Android 7.1.1 Nougat as a part of the Google Play Services 10.2. When you have multiple devices logged into the same account and one loses a connection, it will automatically offer to share another device’s internet connection. This is an effort so as to automate the process and simplify the setting up of a mobile hotspot.
The mobile hotspot on your phone can be super-useful, but it’s also a little tedious to get it set up when you need it. The Instant Tethering feature links up devices associated with your Google account for acquiring connection. The moment one of the device loses connection, it just asks if you want to tether to your other device without any additional step(uses Bluetooth to negotiate the connection). It even shows you the battery level of your other device. This makes the most sense with a tablet connecting to a phone, but it should theoretically work with any devices as long as one has a cellular connection.
Only phones like the Pixel and Pixel XL will be able to share their internet connection with tablets like the Nexus 9 and Pixel C, not the other way around. One Pixel phone can share its connection with another Pixel phone, however. This service is not rolling out for any other phones except the Nexus and Pixel devices running Android 7.1.1 Nougat, for now and might take a little while more to be introduced to the other segment of android devices.