Google added Wi-Fi hotspot functionality to Android way, way back in the days of Froyo, but it took carriers several years to come to terms with it. Now, you can share your LTE connection with a few restrictions on most devices and carriers. The Pixel 3 and 3 XL do something new with hotspot mode. You can enable the Wi-Fi hotspot while you're connected to Wi-Fi, allowing your phone to share that connection with other devices.
There's no fancy setup process here. Just make sure you're connected to Wi-Fi and then turn on your hotspot. All the settings are the same, but instead of sharing LTE, you'll re-transmit the Wi-Fi connection for other devices to use. We've checked, and this is not available on even the Pixel 2 with Pie, so this is a new feature of the latest Google phones. Samsung also includes a similar feature on its phones, but you have to explicitly enable it. There may be a handful of other Pie phones with this feature, but we can only confirm the Pixels for now.
You could use this feature to share a limited network. For example, a paid hotspot that charges you per device or a hotel room that only lets you have one or two devices on the free Wi-Fi. Depending on the strength of the Pixel's hotspot (which I haven't tested extensively) you could also use it as a repeater to extend a weak Wi-Fi signal.
Some other devices, too
Based on reader reports and a handful of tips we've received, several other phones also have this functionality, including recent Samsung flagships, Essential's PH-1, some LG flagships, and a smattering of Xiaomi and Huawei phones. As always, YMMV, but this isn't a Pixel-only feature (though it is new for the series).