Google Pixel and Nexus devices recently received a new ability called Instant Tethering, wherein they can automatically jump off of each other's internet connections if they're signed into the same Google account, and that feature is now rolling out to more users of qualifying devices on qualifying firmware via a server-side update to Google Play Services. The feature began rolling out last month to a limited crop of devices for testing, and officially begins seeding to all qualifying users today. Instant tethering is on by default as long as Bluetooth is on, and will automatically seek out qualifying devices. The Pixel, Pixel XL, Nexus 5X, Nexus 6P, and Nexus 6 can all give their connection to other Nexus devices, so long as they're running Android 7.1.1 (Nougat) and up, while those devices (and the Pixel C and Nexus 9) can receive the automated dose of internet connectivity if they are running Android 6.0 (Marshmallow) and up. This means that a Pixel owner, for example, can give their Nexus 9 a jolt of Wi-Fi without having to log in on the target device.
In order to configure the feature on a host device, head to the "Personal" section of your device's settings app, then to "Google", and then to "Instant Tethering". From there, you can toggle giving and receiving a connection automatically on and off, as well as view a list of nearby qualifying devices that you can sap a connection from or give a connection to. When a target device gets close to a qualifying host device, a "Wi-Fi Hotspot Available" notification will pop up. Simply select the right host device and tap "Connect" to start the connection sharing. The host device will display a notification the entire time that it's tethering. They can either tap the notification, or leave the connection inactive for ten minutes in order to disconnect.
While this process is a bit easier and more secure than configuring a regular Wi-Fi hotspot from your phone and signing in on the target device, keep in mind that you are still tethering, which carries many caveats. For starters, some carriers either don't allow tethering in any form, or give a certain data limit. Tethering also tends to use up a device's battery fairly quickly, and can even cause greater operational heat than normal. This means that those who plan to use tethering for an extended period should probably plug their phone in, and any phone that's tethering should be monitored for signs of overheating.